[mips] Restrict tail call optimization

[mips] Restrict tail call optimization

The tail call optimization was being used without proper consideration of
ABI requirements for saving and restoring the GP. This patch restricts tail
call optimization to functions within the same translation unit.

Reviewers: vkalintiris

Differential Revision: https://reviews.llvm.org/D24763

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287505 91177308-0d34-0410-b5e6-96231b3b80d8