[X86] Custom legalize v2i32 gathers via widening rather than promoting.

The default legalization for v2i32 is promotion to v2i64. This results in a gather that reads 64-bit elements rather than 32. If one of the elements is near a page boundary this can cause an illegal access that can fault.

We also miscalculate the scale for the gather which is an even worse problem, but we probably could have found a separate way to fix that.

llvm-svn: 319521