[X86] Pre-truncate gather/scatter indices that have element sizes larger than...

[X86] Pre-truncate gather/scatter indices that have element sizes larger than 64-bits before Legalize.

The wider element type will normally cause legalize to try to split and scalarize the gather/scatter, but we can't handle that. Instead, truncate the index early so the gather/scatter node is insulated from the legalization.

This really shouldn't happen in practice since InstCombine will normalize index types to the same size as pointers.

llvm-svn: 318452