Add code which does sign/zero extend.
I've tried to ignore the bits that aren't supposed to be in this patch but it's possible I've ignored something important. Could you refresh the patch without the noise?
Please make it clear in the summary (and the title) that this is for MIPS FastISel? It won't be obvious to others when you come to commit
Is the testcase loadstoreconv.ll? I think there should also be a simpler test case that is just sign/zero extension by itself.
The function name and implementation suggests that only sign extensions are supported. Zero extensions are in EmitIntZExt()
The same is true for MIPS and the code below looks right but the comment references ARM.
You'll need to run clang-format on the code though