[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