[committed][RISC-V][PR target/120333] Remove bogus bext pattern

[committed][RISC-V][PR target/120333] Remove bogus bext pattern

I goof'd when doing analysis of missed bext cases.  For the shift into the sign
bit, then shift into the low bit case (thankfully the least common), I got it
in my brain that the field is at the left shift count.   It's actually at
word_size - 1 - left shift count.

One the subtraction is included, it's no longer profitable to turn those cases
into bext.  Best case scenario would be sub+bext, but we can just as easily use
sll+srl which fuses in some designs into a single op.

So this patch removes those two patterns, adjusts the existing testcase and
adds the new execution test.

Given it's a partial reversion and has passed in my tester, I'm going to go
ahead and push it to the trunk rather than waiting for upstream CI.

	PR target/120333
gcc/
	* config/riscv/bitmanip.md: Remove bext formed from left+right
	shift patterns.

gcc/testsuite/

	* gcc.target/riscv/pr114512.c: Update expected output.
	* gcc.target/riscv/pr120333.c: New test.