[LV] Add support for insertelt/extractelt processing during type truncation

Summary:
While shrinking types according to the required bits, we can
encounter insert/extract element instructions. This will cause us to
reach an llvm_unreachable statement.

This change adds support for truncating insert/extract element
operations, and adds a regression test.

Reviewers: jmolloy

Subscribers: mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D17078

llvm-svn: 260893