[MIPS GlobalISel] Fix check for void return during lowerCall

Void return used to have unsigned with value 0 for virtual register
but with addition of Register class and changes to arguments to lowerCall
this is no longer valid.
Check for void return by inspecting the Ty field in OrigRet.

Differential Revision: https://reviews.llvm.org/D65321

llvm-svn: 367107