tree-optimization/120031 - CTZ pattern matching fails a case

tree-optimization/120031 - CTZ pattern matching fails a case

This PR is about the pattern matching in tree-ssa-forwprop.cc not
working for the fallback implementation in ZSTD which uses a cast
aroud the negation of the value to be tested.  There's a pattern
eliding casts in (T')-(T)x already but that only covered an
inner widening conversion.  The following extends this to other
conversions given the negation will then be carried out in an
unsigned type.

	PR tree-optimization/120031
	* match.pd ((nop_outer_cast)-(inner_cast)var -> -(outer_cast)(var)):
	Allow inner conversions that are not widenings when the outer
	type is unsigned.

	* gcc.target/i386/pr120031.c: New testcase.