[PATCH][X86] __builtin_ctz/clz sometimed defined for zero input

[Robinson, Paul]

In general, count-zeros instructions are undefined for a zero input value.
However the X86 TZCNT and LZCNT instructions do return the bit-width on a
zero input, so make Clang tell LLVM so.
One quirk is that these instructions aren't necessarily both defined, so
also create a separate predicate so we can do the right thing for all CPUs.
--paulr

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