c-family: Fix problems with blender and PPC from PR 40752 patch.
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.