[MIPS, test] Fix use of undef FileCheck var

[thopre]

Summary

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.

[dsanders]

LGTM

llvm/test/CodeGen/Mips/sr1.ll

entry:
   %call = call float @xf()
   store float %call, float* @f, align 4
   ret void
 ; CHECK:  .ent  foo3
 ; CHECK:  save  $16, $17, $ra, $18, [[FS:[0-9]+]]
 ; CHECK:  restore $16, $17, $ra, $18, [[FS]]
 ; CHECK:  .end  foo3
 ; NEG:  .ent  foo3
-; NEG-NOT:  save  $16, $17, $ra, $18, [[FS:[0-9]+]] # 16 bit inst
-; NEG-NOT:  restore $16, $17, $ra, $18, [[FS]] # 16 bit inst
+; NEG-NOT:  save  $16, $17, $ra, $18, {{[0-9]+}} # 16 bit inst
+; NEG-NOT:  restore $16, $17, $ra, $18, {{[0-9]+}} # 16 bit inst
 ; NEG:  .end  foo3
 }
 
 declare float @xf() #1
 
 attributes #0 = { nounwind "less-precise-fpmad"="false" "frame-pointer"="all" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
 attributes #1 = { "less-precise-fpmad"="false" "frame-pointer"="all" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }