[mips][dsp] Modify repl.ph to accept signed immediate values

Changed immediate type for repl.ph from uimm10 to simm10 as per the specs.
Repl.qb still accepts uimm8. Both instructions now mimic the behaviour of
GNU as.

Patch by Stefan Maksimovic.

Differential Revision: https://reviews.llvm.org/D33594

llvm-svn: 304918