[GlobalISel][IRTranslator] Split aggregates during IR translation.

We currently handle all aggregates by creating one large LLT, and letting the
legalizer deal with splitting them up. However using this approach means that
we can't support big endian code correctly.

This patch changes the way that the IRTranslator deals with aggregate values,
by splitting them up into their constituent element values. To do this, parts
of the translator need to be modified to deal with multiple VRegs for a single
Value.

A new Value to VReg mapper is introduced to help keep compile time under
control, currently there is no measurable impact on CTMark despite the extra
code being generated in some cases.

Patch is based on the original work of Tim Northover.

Differential Revision: https://reviews.llvm.org/D46018

llvm-svn: 332449