phiopt: Improve minmax optimization [PR95699]

phiopt: Improve minmax optimization [PR95699]

As discussed in the PR, the
x < 0x80000000U to (int) x >= 0
optimization stands in the way of minmax_replacement optimization,
so for comparisons with most of the constants it works well, but when the
above mentioned optimization triggers, it is unable to do it.
The match.pd (cond (cmp (convert? x) c1) (op x c2) c3) -> (op (minmax x c1) c2)
optimization is able to look through that and this patch
teaches minmax_replacement about it too.

2020-06-18  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/95699
	* tree-ssa-phiopt.c (minmax_replacement): Treat (signed int)x < 0
	as x > INT_MAX and (signed int)x >= 0 as x <= INT_MAX.  Move variable
	declarations to the statements that set them where possible.

	* gcc.dg/tree-ssa/pr95699.c: New test.