[RISC-V][PR middle-end/114512] Recognize more bext idioms for RISC-V

[RISC-V][PR middle-end/114512] Recognize more bext idioms for RISC-V

This is Shreya's next chunk of work.  When I was looking for good bugs for her
to chase down I cam across PR114512.  While the bug isn't necessarily a RISC-V
specific bug, its testcases did show how we were failing to recognize certain
bit extraction idioms and how the lispy nature of RTL allows us to tackle these
issues in the combiner.

First, the bit position may be masked.  The RISC-V port does not define
SHIFT_COUNT_TRUNCATED for valid reasons.  So if we want to optimize away a mask
that matches what the hardware will do, we need suitable insns that include
that explicit masking.

In addition to needing to incorporate masking, the masking may happen in a
subword mode.  So we need to recognize the mask wrapped in a zero extension.

Those two captured the most common cases.

We can also have a single bit extraction implemented as a left shift of the bit
into the sign bit, then a right shift by the size of a word - 1.  These are
less common, but we did cover the case derived from the upstream bug report as
well as one class seen reviewing the instruction stream for spec2017.

Finally, extracting a single bit at a variable position from a constant as seen
with some regularity in spec2017.  In that scenario, combine's chosen split
point wasn't ideal (I forget what it selected, but it definitely wasn't
helpful).  So we've got a new splitter for this case as well.

Earlier versions of this have gone through my tester as well as a bootstrap and
regression cycle.  This version has just gone through a cycle in my tester (but
missed today's bootstrap cycle).

Waiting on the upstream pre-commit tester to render its verdict, but the plan
is to commit on Shreya's behalf once that's clean.

    Co-authored-by: Jeff Law  <jlaw@ventanamicro.com>
	PR middle-end/114512
gcc/
	* config/riscv/bitmanip.md (bext* patterns): New patterns for
	bext recognition plus splitter for extracting variable bit from
	a constant.
	* config/riscv/predicates.md (bitpos_mask_operand): New predicate.

gcc/testsuite/
	* gcc.target/riscv/pr114512.c: New test.