Make test more resilient to FastIsel changes. NFC.

Currently FastISel doesn't know how to select vector bitcasts.
During instruction selection, fast-isel always falls back to SelectionDAG 
every time it encounters a vector bitcast.
As a consequence of this, all the 'packed vector shift by immedate count'
test cases in avx2-builtins.c are optimized by the DAGCombiner.
In particular, the DAGCombiner would always fold trivial stack loads of
constant shift counts into the operands of packed shift builtins.

This behavior would start changing as soon as I reapply revision 249121.
That revision would teach x86 fast-isel how to select bitcasts between vector
types of the same size.

As a consequence of that change, fast-isel would less often fall back to
SelectionDAG. More importantly, DAGCombiner would no longer be able to 
simplify the code by folding the stack reload of a constant.

No functional change.

llvm-svn: 249142