[mips] MIPS-IV is broadly the same as MIPS64 so duplicate all -mcpu=mips64 tests with -mcpu=mips4 as a starting point

Summary:
Two exceptions to this:
  test/CodeGen/Mips/octeon.ll
  test/CodeGen/Mips/octeon_popcnt.ll
these test extensions to MIPS64

One test is altered for MIPS-IV:
  test/CodeGen/Mips/mips64countleading.ll
    Tests dclo/dclz which were added in MIPS64. The MIPS-IV version tests
    that dclo/dclz are not emitted.

Four tests fail and are not in this patch:
  test/CodeGen/Mips/abicalls.ll
  test/CodeGen/Mips/fcopysign-f32-f64.ll
  test/CodeGen/Mips/fcopysign.ll
  test/CodeGen/Mips/stack-alignment.ll

Depends on D3343

Reviewers: matheusalmeida, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3344