[SelectionDAG] Extend base addressing modes supported by MGATHER/MSCATTER

Summary:
Before this patch MGATHER/MSCATTER is capable of representing all
common addressing modes, but only when illegal types are used.
This patch adds an IndexType property so more representations
are available when using legal types only.

Original modes:
 vector of bases
 base + vector of signed scaled offsets

New modes:
 base + vector of signed unscaled offsets
 base + vector of unsigned scaled offsets
 base + vector of unsigned unscaled offsets

The current behaviour of addressing modes for gather/scatter remains
unchanged.

Patch by Paul Walker.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D65636

llvm-svn: 368008