[InstCombine] try to turn shuffle into insertelement

shuffle (insert ?, Scalar, IndexC), V1, Mask --> insert V1, Scalar, IndexC'

The motivating case is at least a couple of steps away: I noticed that
SLPVectorizer does not analyze shuffles as well as sequences of 
insert/extract in PR34724:
https://bugs.llvm.org/show_bug.cgi?id=34724
...so SLP may fail to vectorize when source code has shuffles to start 
with or instcombine has converted insert/extract to shuffles.

Independent of that, an insertelement is always a simpler op for IR 
analysis vs. a shuffle, so we should transform to insert when possible.

I don't think there's any codegen concern here - if a target can't insert 
a scalar directly to some fixed element in a vector (x86?), then this 
should get expanded to the insert+shuffle that we started with.

Differential Revision: https://reviews.llvm.org/D53507

llvm-svn: 345607