PR14055: Implement support for sub-vector operations in SROA.

PR14055: Implement support for sub-vector operations in SROA.

Now if we can transform an alloca into a single vector value, but it has
subvector, non-element accesses, we form the appropriate shufflevectors
to allow SROA to proceed. This fixes PR14055 which pointed out a very
common pattern that SROA couldn't handle -- mixed vec3 and vec4
operations on a single alloca.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168418 91177308-0d34-0410-b5e6-96231b3b80d8