InstCombine: extract instead of shuffle when performing vector/array type punning

Summary: SROA generates code that isn't quite as easy to optimize and contains unusual-sized shuffles, but that code is generally correct. As discussed in D7487 the right place to clean things up is InstCombine, which will pick up the type-punning pattern and transform it into a more obvious bitcast+extractelement, while leaving the other patterns SROA encounters as-is.

Test Plan: make check

Reviewers: jvoung, chandlerc

Subscribers: llvm-commits
llvm-svn: 230560