[GlobalISel][Utils] Add a getConstantVRegVal variant that looks through instrs

getConstantVRegVal used to only look for G_CONSTANT when looking at
unboxing the value of a vreg. However, constants are sometimes not
directly used and are hidden behind trunc, s|zext or copy chain of
computation.

In particular this may be introduced by the legalization process that
doesn't want to simplify these patterns because it can lead to infine
loop when legalizing a constant.

To circumvent that problem, add a new variant of getConstantVRegVal,
named getConstantVRegValWithLookThrough, that allow to look through
extensions.

Differential Revision: https://reviews.llvm.org/D59227

llvm-svn: 356116