[mips] Fix unsigned/signed type error

MipsFastISel uses a a class to represent addresses with a signed member
to represent the offset. MipsFastISel::emitStore, emitLoad and computeAddress
all treated the offset as being positive. In cases where the offset was
actually negative and a frame pointer was used, this would cause the constant
synthesis routine to crash as it would generate an unexpected instruction
sequence when frame indexes are replaced.

Reviewers: vkalintiris

Differential Revision: https://reviews.llvm.org/D26192

llvm-svn: 287099