[mips] Simplify ordering of range checked immediate classes.

Summary:
With the addition of checks to ensure that operands have a strict ordering
it has become tricky to manage the order in the way I originally intended.

This patch linearizes the ordering which simplifies the implementation but
requires an order that is arbitrary in places. Here are some examples:
* uimm4 < uimm5 < uimm6
* simm4 < uimm4 < simm5 < uimm5
* uimm5 < uimm5_plus1 (1..32) < uimm5_plus32 (32..63) < uimm6
  The term 'superset' starts to break down here since the *_plus* classes
  are not true supersets of uimm5 (but they are still subsets of uimm6).
* uimm5 < uimm5_64, and uimm5 < vsplat_uimm5
  This is entirely arbitrary. We need an ordering and what we pick is
  unimportant since only one is possible for a given mnemonic.

Reviewers: vkalintiris

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D17723

llvm-svn: 263423