[AMDGPU] lowerEXTRACT_VECTOR_ELT - fold from a SCALAR_TO_VECTOR source

As suggested by @foad on D124839

If we're extracting a vector element that originally came from a scalar_to_vector, then avoid the bitcasting of a vector type and perform the shift masking on the (any-extended) scalar source directly, making use of the fact that the upper elements of a scalar_to_vector are all undef.

Differential Revision: https://reviews.llvm.org/D125173