IndVarSimplify: Address review comments for r217102

No functional change intended, just some cleanups and comments added.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217115 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Teach the new vector shuffle lowering about the zero masking
abilities of INSERTPS which are really powerful and come up in very
important contexts such as forming diagonal matrices, etc.

With this I ended up being able to remove the somewhat weird helper
I added for INSERTPS because we can collapse the entire state to a no-op
mask. Added a bunch of tests for inserting into a zero-ish vector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217117 91177308-0d34-0410-b5e6-96231b3b80d8