Subzero, MIPS32: UnimplementedError removed from most common crashers

Subzero, MIPS32: UnimplementedError removed from most common crashers

UnimplementedError is removed from three functions because it causes
crashes in almost every test from tests_lit/llvm2ice_tests.

emitVariable appears as correct, UnimplementedError may be there by mistake.

lowerConstants is unimplemented, but its obvious without UnimeplementedError.

lowerJumpTables is unimplemented, but its obvious without UnimeplementedError.

We comment these three anyway while changing and testing, so removing them will
not affect final results anyway.

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2086423002 .

Patch from Srdjan Obucina <Srdjan.Obucina@imgtec.com>.