[InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 1

[InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 1

This is the main patch for converting a truncated scalar that is
inserted into a vector to bitcast+shuffle. We could go either way
on patterns like this, but this direction will allow collapsing a
pair of these sequences on the motivating example from issue

The patch is split into 3 parts to make it easier to see the
progression of tests diffs. We allow inserting/shuffling into a
different size vector for flexibility, so there are several test
variations. The length-changing is handled by shortening/padding
the shuffle mask with undef elements.

In part 1, handle the basic pattern:
inselt undef, (trunc T), IndexC --> shuffle (bitcast T), IdentityMask

Proof for the endian-dependency behaving as expected:
https://alive2.llvm.org/ce/z/BsA7yC

The TODO items for handling shifts and insert into an arbitrary base
vector value are implemented as follow-ups.

Differential Revision: https://reviews.llvm.org/D138872