tree-optimization/95393 - fold MIN/MAX_EXPR generated by phiopt

This makes sure to fold generated stmts so they do not survive
until RTL expansion and cause awkward code generation.

2020-05-29  Richard Biener <rguenther@suse.de>

	PR tree-optimization/95393
	* tree-ssa-phiopt.c (minmax_replacement): Use gimple_build
	to build the min/max expression so we simplify cases like
	MAX(0, s) immediately.

	* gcc.dg/tree-ssa/phi-opt-21.c: New testcase.
	* g++.dg/vect/slp-pr87105.cc: Adjust.