[X86][Disassembler] Clamp index to 4-bits when decoding GPR registers.

A 5-bit value can occur when EVEX.X is 0 due to it being used to extend modrm.rm to encode XMM16-31. But if modrm.rm instead encodes a GPR, the Intel documentation says EVEX.X should be ignored so just mask it to 4 bits once we know its a GPR.

llvm-svn: 333725