match.pd: Fix up __builtin_*_overflow arg demotion [PR102207]

match.pd: Fix up __builtin_*_overflow arg demotion [PR102207]

My earlier patch to demote arguments of __builtin_*_overflow unfortunately
caused a wrong-code regression.  The builtins operate on infinite precision
arguments, outer_prec > inner_prec signed -> signed, unsigned -> unsigned
promotions there are just repeating the sign or 0s and can be demoted,
similarly unsigned -> signed which also is repeating 0s, but as the
testcase shows, signed -> unsigned promotions need to be preserved (unless
we'd know the inner arguments can't be negative), because for negative
numbers such promotion sets the outer_prec -> inner_prec bits to 1 bit the
bits above that to 0 in the infinite precision.

So, the following patch avoids the demotions for the signed -> unsigned
promotions.

2021-09-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102207
	* match.pd: Don't demote operands of IFN_{ADD,SUB,MUL}_OVERFLOW if they
	were promoted from signed to wider unsigned type.

	* gcc.dg/pr102207.c: New test.