gcc: Add vec_select -> subreg RTL simplification

gcc: Add vec_select -> subreg RTL simplification

Add a new RTL simplification for the case of a VEC_SELECT selecting
the low part of a vector. The simplification returns a SUBREG.

The primary goal of this patch is to enable better combinations of
Neon RTL patterns - specifically allowing generation of 'write-to-
high-half' narrowing intructions.

Adding this RTL simplification means that the expected results for a
number of tests need to be updated:
* aarch64 Neon: Update the scan-assembler regex for intrinsics tests
  to expect a scalar register instead of lane 0 of a vector.
* aarch64 SVE: Likewise.
* arm MVE: Use lane 1 instead of lane 0 for lane-extraction
  intrinsics tests (as the move instructions get optimized away for
  lane 0.)

This patch also adds new code generation tests to
narrow_high_combine.c to verify the benefit of this RTL
simplification.

gcc/ChangeLog:

2021-06-08  Jonathan Wright  <jonathan.wright@arm.com>

	* combine.c (combine_simplify_rtx): Add vec_select -> subreg
	simplification.
	* config/aarch64/aarch64.md (*zero_extend<SHORT:mode><GPI:mode>2_aarch64):
	Add Neon to general purpose register case for zero-extend
	pattern.
	* config/arm/vfp.md (*arm_movsi_vfp): Remove "*" from *t -> r
	case to prevent some cases opting to go through memory.
	* cse.c (fold_rtx): Add vec_select -> subreg simplification.
	* rtl.c (rtvec_series_p): Define predicate to determine
	whether a vector contains a linear series of integers.
	* rtl.h (rtvec_series_p): Define.
	* rtlanal.c (vec_series_lowpart_p): Define predicate to
	determine if a vector selection is equivalent to the low part
	of the vector.
	* rtlanal.h (vec_series_lowpart_p): Define.
	* simplify-rtx.c (simplify_context::simplify_binary_operation_1):
	Add vec_select -> subreg simplification.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/extract_zero_extend.c: Remove dump scan
	for RTL pattern match.
	* gcc.target/aarch64/narrow_high_combine.c: Add new tests.
	* gcc.target/aarch64/simd/vmulx_laneq_f64_1.c: Update
	scan-assembler regex to look for a scalar register instead of
	lane 0 of a vector.
	* gcc.target/aarch64/simd/vmulxd_laneq_f64_1.c: Likewise.
	* gcc.target/aarch64/simd/vmulxs_lane_f32_1.c: Likewise.
	* gcc.target/aarch64/simd/vmulxs_laneq_f32_1.c: Likewise.
	* gcc.target/aarch64/simd/vqdmlalh_lane_s16.c: Likewise.
	* gcc.target/aarch64/simd/vqdmlals_lane_s32.c: Likewise.
	* gcc.target/aarch64/simd/vqdmlslh_lane_s16.c: Likewise.
	* gcc.target/aarch64/simd/vqdmlsls_lane_s32.c: Likewise.
	* gcc.target/aarch64/simd/vqdmullh_lane_s16.c: Likewise.
	* gcc.target/aarch64/simd/vqdmullh_laneq_s16.c: Likewise.
	* gcc.target/aarch64/simd/vqdmulls_lane_s32.c: Likewise.
	* gcc.target/aarch64/simd/vqdmulls_laneq_s32.c: Likewise.
	* gcc.target/aarch64/sve/dup_lane_1.c: Likewise.
	* gcc.target/aarch64/sve/extract_1.c: Likewise.
	* gcc.target/aarch64/sve/extract_2.c: Likewise.
	* gcc.target/aarch64/sve/extract_3.c: Likewise.
	* gcc.target/aarch64/sve/extract_4.c: Likewise.
	* gcc.target/aarch64/sve/live_1.c: Update scan-assembler regex
	cases to look for 'b' and 'h' registers instead of 'w'.
	* gcc.target/arm/crypto-vsha1cq_u32.c: Update scan-assembler
	regex to reflect lane 0 vector extractions being simplified
	to scalar register moves.
	* gcc.target/arm/crypto-vsha1h_u32.c: Likewise.
	* gcc.target/arm/crypto-vsha1mq_u32.c: Likewise.
	* gcc.target/arm/crypto-vsha1pq_u32.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_f16.c: Extract
	lane 1 as the moves for lane 0 now get optimized away.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_f32.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_s16.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_s32.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_s8.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_u16.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_u32.c: Likewise.
	* gcc.target/arm/mve/intrinsics/vgetq_lane_u8.c: Likewise.