[mlir][StandardToSPIRV] Emulate bitwidths not supported for load op.

Summary:
The current implementation in SPIRVTypeConverter just unconditionally turns
everything into 32-bit if it doesn't meet the requirements of extensions or
capabilities. In this case, we can load a 32-bit value and then do bit
extraction to get the value.

Differential Revision: https://reviews.llvm.org/D78974