blender in SPEC is built with -funsigned-char, which is also the default on
PPC, and exposed -Wsign-conversion issues that weren't seen by the x86_64
testsuite.  In blender we were complaining about operands to an expression
that we didn't't previously complain about as a whole.  So only check
operands after we check the whole expression.  Also, to fix the PR 40752
testcases on -funsigned-char targets, don't consider -Wsign-conversion for
the second operand of PLUS_EXPR, especially since fold changes
"x - 5" to "x + (-5)".  And don't use SCHAR_MAX with plain char.

	PR testsuite/93391 - PR 40752 test fails with unsigned plain char.
	PR c++/40752
	* c-warn.c (conversion_warning): Check operands only after checking
	the whole expression.  Don't check second operand of + for sign.