[mips] Make isel select the correct DEXT variant up front.

[dsanders]

Summary

Previously, it would always select DEXT and substitute any invalid matches
for DEXTU/DEXTM during MipsMCCodeEmitter::encodeInstruction(). This works
but causes problems when adding range checked immediates to IAS.

Now isel selects the correct variant up front.

[dsanders]

ping

[vkalintiris]

LGTM, with a small change as described in the in-line comment.

lib/Target/Mips/MipsInstrInfo.td

808–816

The names imply that we add a constant to the zext'd value, but we perform a subtraction instead.

[dsanders]

lib/Target/Mips/MipsInstrInfo.td

808–816

The intent is to describe the ranges 1..32, 32..63, and 33..64 which are uimm5's with an offset added. The reason the code performs a subtraction is that the predicate must undo the offset so that we can test for a normal uimm5.

[vkalintiris]

lib/Target/Mips/MipsInstrInfo.td

808–816

Ok, I was trying to think of a better name such as immZExt5Offset32, but I don't like that either. You can go ahead and commit this then.

[dsanders]

Rebase before commit