[CodeGen] Combine concat_vector of trunc'd scalar to scalar_to_vector.

[CodeGen] Combine concat_vector of trunc'd scalar to scalar_to_vector.

We already do:
  concat_vectors(scalar, undef) -> scalar_to_vector(scalar)
When the scalar is legal.
When it's not, but is a truncated legal scalar, we can also do:
  concat_vectors(trunc(scalar), undef) -> scalar_to_vector(scalar)
Which is equivalent, since the upper lanes are undef anyway.
While there, teach the combine to look at more than 2 operands.

Differential Revision: http://reviews.llvm.org/D8883

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234530 91177308-0d34-0410-b5e6-96231b3b80d8