[mips] handle UImm16_AltRelaxed match type

Currently, UImm16_AltRelaxed match type is not handled in
MatchAndEmitInstruction() function, which may result in
llvm_unreachable() behavior.
This patch adds necessary case for this match type.

Patch by Aleksandar Beserminji.

Differential Revision: https://reviews.llvm.org/D37682

llvm-svn: 313077
1 file changed