[X86][SSE] Add support for extracting target constants from BUILD_VECTOR

In some cases we call getTargetConstantBitsFromNode for nodes that haven't been lowered from BUILD_VECTOR yet

Note: We're getting very close to being able to move most of the constant extraction code from getTargetShuffleMaskIndices into getTargetConstantBitsFromNode

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294746 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed