[committed][PR rtl-optimization/120550] Drop REG_EQUAL note after ext-dce transformation

[committed][PR rtl-optimization/120550] Drop REG_EQUAL note after ext-dce transformation

This bug was found by Edwin's fuzzing efforts on RISC-V, though it likely
affects other targets.

In simplest terms when ext-dce converts an extension into a (possibly
simplified) subreg copy it may make an attached REG_EQUAL note invalid.

In the case Edwin found the note was an extension, but I don't think that would
necessarily always be the case.  The note could have other forms which
potentially need invalidation.  So the safest thing to do is just remove any
attached REG_EQUAL or REG_EQUIV note.

Note adjusting Edwin's testcase in the obvious way to avoid having to interpret
printf output for pass/fail status makes the bug go latent.  That's why no
testcase is included with this patch.

Bootstrapped and regression tested on x86_64.  Obviously also verified it fixes
the testcase Edwin filed.

This is a good candidate for cherry-picking to the gcc-15 release branch after
simmering on the trunk a bit.

	PR rtl-optimization/120550
gcc/
	* ext-dce.cc (ext_dce_try_optimize_insn): Drop REG_EQUAL/REG_EQUIV
	notes on modified insns.