[X86] Properly disassemble gather/scatter instructions where xmm4/ymm4/zmm4 are used as the index.

These encodings correspond to the cases in the normal encoding scheme where there is no index and our modrm reading code initially decodes it as such. The VSIB handling code tried to compensate for this, but failed to add the base needed to make later code do the right thing.

Fixes PR37712.

llvm-svn: 334121