[mips] Change lwl and lwr in inlineasm_constraint.ll to lw

Summary:
lwl and lwr are not available in MIPS32r6/MIPS64r6. The purpose of the test
is to check that the '$1' expands to '0($x)' rather than to test something related
to the lwl or lwr instructions so we can simply switch to lw.

Depends on D3842

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3844