X86 requires an immediate shift value to fit within 8 bits.  It's undefined LLVM behavior if it doesn't (or in fact if the value exceeds the number of bits in the type width), but at least we can produce valid x86 instructions.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4316
R=ascull@google.com

Review URL: https://codereview.chromium.org/1339603006 .