PR middle-end/103406: Check for Inf before simplifying x-x.

PR middle-end/103406: Check for Inf before simplifying x-x.

This is a simple one line fix to the regression PR middle-end/103406,
where x - x is being folded to 0.0 even when x is +Inf or -Inf.
In GCC 11 and previously, we'd check whether the type honored NaNs
(which implicitly covered the case where the type honors infinities),
but my patch to test whether the operand could potentially be NaN
failed to also check whether the operand could potentially be Inf.

2021-11-25  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/103406
	* match.pd (minus @0 @0): Check tree_expr_maybe_infinite_p.

gcc/testsuite/ChangeLog
	PR middle-end/103406
	* gcc.dg/pr103406.c: New test case.