[DAGCombine] Truncate BUILD_VECTOR operators if necessary when constant folding vectors

[DAGCombine] Truncate BUILD_VECTOR operators if necessary when constant folding vectors

Summary:
The BUILD_VECTOR node will truncate its operators to match the
type. We need to take this into account when constant folding -
we need to perform a truncation before constant folding the elements.
This is because the upper bits can change the result, depending on
the operation type (for example this is the case for min/max).

This change also adds a regression test.

Reviewers: jmolloy

Subscribers: jmolloy, llvm-commits

Differential Revision: http://reviews.llvm.org/D12697

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247265 91177308-0d34-0410-b5e6-96231b3b80d8