[PATCH] [mips] Account for endianess when expanding BuildPairF64/ExtractElementF64 nodes.

[Vasileios Kalintiris]

Please, note that with these changes, the prefixes 32R2-FP64A-BE & 32R2-FP64A-LE in the file test/CodeGen/Mips/fp64a.ll
expect the same output. If the changes are fine, I will merge them under a new single check prefix in another review request.

http://reviews.llvm.org/D5753