[mips] Keep the parameter list of Filler::searchRange() consistent. NFC.

Summary:
Move the "Filler" parameter to the end of the parameter list as it is,
conceptually, the only output parameter of that function.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7726

llvm-svn: 231239