[mips][mips64r6] Improve tests affected by the changes to multiplies and divides

Summary:
MIPS32r6/MIPS64r6 support has not been added yet.

inlineasm-cnstrnt-reg.ll:
  Explicitly specify the CPU since it will not work on MIPS32r6/MIPS64r6
  when -integrated-as is the default. We can't change the mnemonic since the
  LO register is an implicit def of mtlo and MIPS32r6/MIPS64r6 has no
  instructions that use LO.

2008-08-01-AsmInline.ll:
  Explicitly specify the CPU since MIPS32r6/MIPS64r6 will correctly emit
  different code and this is a regression test.

mips64instrs.ll and mips64muldiv.ll
  Check registers and the way the multiply is used in m1

divrem.ll
  Check registers and use multiple filecheck prefixes to limit redundancy

Reviewers: vmedic, jkolek, zoran.jovanovic, matheusalmeida

Reviewed By: matheusalmeida

Subscribers: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3894

llvm-svn: 210656