[mips] Expand pseudo instructions before they are emitted in
MipsCodeEmitter.cpp.

JALR and NOP are expanded by function emitPseudoExpansionLowering, which is not
called when the old JIT is used.

This fixes the following tests which have been failing on
llvm-mips-linux builder:

LLVM :: ExecutionEngine__2003-01-04-LoopTest.ll
LLVM :: ExecutionEngine__2003-05-06-LivenessClobber.ll
LLVM :: ExecutionEngine__2003-06-04-bzip2-bug.ll
LLVM :: ExecutionEngine__2005-12-02-TailCallBug.ll
LLVM :: ExecutionEngine__2003-10-18-PHINode-ConstantExpr-CondCode-Failure.ll
LLVM :: ExecutionEngine__hello2.ll
LLVM :: ExecutionEngine__stubs.ll
LLVM :: ExecutionEngine__test-branch.ll
LLVM :: ExecutionEngine__test-call.ll
LLVM :: ExecutionEngine__test-common-symbols.ll
LLVM :: ExecutionEngine__test-loadstore.ll
LLVM :: ExecutionEngine__test-loop.ll