PR middle-end/123236: Simplify (int)((long long)x >> 4)

This patch addresses a code quality regression on x86_64 related to
PR 123236.  That original PR (and the related PR 101266) concern tree
level optimizations, where this problem should also be fixed, but it
also reveals a regression in the RTL optimizers.

A motivating test case (on x86_64) is:

int bar(int a) {
  long long t = a;
  return t >> 4;
}

Currently -O2 generates a 64-bit shift:
bar:    movslq  %edi, %rax
        sarq    $4, %rax
        ret

with this patch we now generate a 32-bit shift:
bar:    movl    %edi, %eax
        sarl    $4, %eax
        ret

The underlying cause of the RTL-level regression is that some
RTL expressions that were previously expressed as {ZERO,SIGN}_EXTEND
are now sometimes represented as the equivalent {ZERO,SIGN}_EXTRACT,
and that not all simplifications of TRUNCATE({ZERO,SIGN}_EXTEND) are
implemented for TRUNCATE({ZERO,SIGN}_EXTRACT).

Thanks to Segher, simplify_rtx does handle some truncations of extracts,
see https://gcc.gnu.org/pipermail/gcc-patches/2016-November/463629.html
but unfortunately this code (and subsequent tweaks) doesn't quite match
the cases that appear here.

2026-07-15  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/123236
	* simplify-rtx.cc (simplify_context::simplify_truncation): Handle
	cases where a ZERO_EXTRACT or SIGN_EXTRACT has a different mode
	to (but at least as wide as) its first operand.

gcc/testsuite/ChangeLog
	PR rtl-optimization/123236
	* gcc.target/i386/pr123236-1.c: New test case.