[mips] Make unalignedload.ll test stricter and easier to modify for MIPS32r6/MIPS64r6

[mips] Make unalignedload.ll test stricter and easier to modify for MIPS32r6/MIPS64r6

Summary:
* Split into two functions, one to test each struct.
* R0 and R2 must be defined by an lw with a %got reference to the correct
  symbol.
* Test for $4 (first argument) where appropriate instead of accepting any
  register.
* Test that the two lbu's are correctly combined into $4

Depends on D3844

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3845

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209424 91177308-0d34-0410-b5e6-96231b3b80d8