[mips][mips64r6] [sl][duw]xc1 are not available on MIPS32r6/MIPS64r6

Summary:
Folded mips64-fp-indexed-ls.ll into fp-indexed-ls.ll. To do so, the zext's in
mips64-fp-indexed-ls.ll were changed to implicit sign extensions (performed
by getelementptr). This does not affect the purpose of the test.

Depends on D4004

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4110