dc0e258fe4
Previously, registers and subregisters mapped to the same Dwarf encoding. We don't really have any way to refer to subregisters directly from Dwarf, the expression emitter should instead use DW_OPs to stencil out the subregister from the whole register. This was also confusing tools that need to map back to the llvm reg (e.g. dwarfdump), since getLLVMRegNum() would arbitrarily return the _LO16 register.