[MIPS, test] Fix use of undef FileCheck var

LLVM test CodeGen/Mips/sr1.ll tries to check for the absence of a
sequence of instructions with several CHECK-NOT with one of those
directives using a variable defined in another. However CHECK-NOT are
checked independently so that is using a variable defined in a pattern
that should not occur in the input.

This commit removes the definition and uses of variable to check each
line independently, making the check stronger than the current one.

Reviewed By: dsanders

Differential Revision: https://reviews.llvm.org/D99776