[mlir][spirv] Fix bitcast input order for UnifyAliasedResourcePass

spv.bitcast from a vector to a scalar expects the lower-numbered
components of the the vector to map to the lower-ordered bits of
the scalar. That actually already matches how little endian stores
data in the memory. So we just need to read and push to the back
of the vector sequentially.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D128473