Extend PEI to emit a DWARF expression for StackOffsets that have
a fixed and scalable component. This means the expression that needs
to be added is either:
<base> + offset
or:
<base> + offset + scalable_offset * scalereg
where for SVE, the scale reg is the Vector Granule Dwarf register, which
encodes the number of 64bit 'granules' in an SVE vector and which
the debugger can evaluate at runtime.
We currently don't allow hardcoded registers inside of a DIExpression, The clean solution for this would be to rebase this on the patch that adds support for DW_OP_LLVM_argX / DBG_VALUE_LIST.