[MIPS GlobalISel] Lower SRet pointer arguments

Instead of returning structure by value clang usually adds pointer
to that structure as an argument. Pointers don't require special
handling no matter the SRet flag. Remove unsuccessful exit from
lowerCall for arguments with SRet flag if they are pointers.

Differential Revision: https://reviews.llvm.org/D67179

llvm-svn: 371054