Adjust bit cast instruction filter for DXIL Prepare pass (#142678)
This PR addresses a specific edge case when deciding whether or not to
produce a bitcast instruction.
Specifically, when the given instruction is a global array, the element
type of the array wasn't correctly compared to the return type. In this
specific case, if the types are equal, a bitcast shouldn't be created,
but it was.
This PR checks to see if the element type of the array is the same as
the return type, and if it is, it doesn't create a bitcast instruction.
Fixes https://github.com/llvm/llvm-project/issues/139013