AArch64: Improve immediate generation

AArch64: Improve immediate generation

Further improve immediate generation by adding support for 2-instruction
MOV/EOR bitmask immediates.  This reduces the number of 3/4-instruction
immediates in SPECCPU2017 by ~2%.

Reviewed-by: Richard Earnshaw <Richard.Earnshaw@arm.com>
gcc/ChangeLog:
	* config/aarch64/aarch64.cc (aarch64_internal_mov_immediate)
	Add support for immediates using MOV/EOR bitmask.

gcc/testsuite:
	* gcc.target/aarch64/imm_choice_comparison.c: Change tests.
	* gcc.target/aarch64/moveor_imm.c: Add new test.
	* gcc.target/aarch64/pr106583.c: Change tests.