[CodeGen] Don't evaluate immediate inlineasm arguments using isICE().

Instead, just EvaluateAsInt().

Follow-up to r239549: rsmith points out that isICE() is expensive;
seems like it's not the right concept anyway, as it fails on
`static const' in C, and will actually trigger the assert below on:
    test/Sema/inline-asm-validate-x86.c

llvm-svn: 239651