Fix Untyped...AccessChainKHR reverse translation (#3940)
The Base Type operand of an untyped access chain is the type being
indexed, not a pointer to it, so it must be used as is.
The patch addresses issues discovered during testing for
https://github.com/llvm/llvm-project/pull/201233
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/706ad87af0c7f31
