PR c/102245: Disable sign-changing optimization for shifts by zero.

PR c/102245: Disable sign-changing optimization for shifts by zero.

Respecting Jakub's suggestion that it may be better to warn-on-valid for
"if (x << 0)" as the author might have intended "if (x < 0)" [which will
also warn when x is _Bool], the simplest way to resolve this regression
is to disable the recently added fold transformation for shifts by zero;
these will be optimized later (elsewhere).  Guarding against integer_zerop
is the simplest of three alternatives; the second being to only apply
this transformation to GIMPLE and not GENERIC, and the third (potentially)
being to explicitly handle shifts by zero here, with an (if cond then else),
optimizing the expression to a convert, but awkwardly duplicating a
more general transformation earlier in match.pd's shift simplifications.

2021-09-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR c/102245
	* match.pd (shift optimizations): Disable recent sign-changing
	optimization for shifts by zero, these will be folded later.

gcc/testsuite/ChangeLog
	PR c/102245
	* gcc.dg/Wint-in-bool-context-4.c: New test case.