c: Fix ICE with -fexcess-precision=standard [PR99136]

c: Fix ICE with -fexcess-precision=standard [PR99136]

The following testcase ICEs on i686-linux, because c_finish_return wraps
c_fully_folded retval back into EXCESS_PRECISION_EXPR, but when the function
return type is void, we don't call convert_for_assignment on it that would
then be fully folded again, but just put the retval into RETURN_EXPR's
operand, so nothing removes it anymore and during gimplification we
ICE as EXCESS_PRECISION_EXPR is not handled.

This patch fixes it by not adding that EXCESS_PRECISION_EXPR in functions
returning void, the return value is ignored and all we need is evaluate any
side-effects of the expression.

2021-02-18  Jakub Jelinek  <jakub@redhat.com>

	PR c/99136
	* c-typeck.c (c_finish_return): Don't wrap retval into
	EXCESS_PRECISION_EXPR in functions that return void.

	* gcc.dg/pr99136.c: New test.