[mips] Use addiu in inline assembly tests since addi is not available in all ISA's

Summary:
This patch is necessary so that they do not fail on MIPS32r6/MIPS64r6 when
-integrated-as is enabled by default and we correctly detect the host CPU.

No functional change since these tests are testing the behaviour of the
constraint used for the third operand rather than the mnemonic.

Depends on D3842

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3843

llvm-svn: 209421