[test, ARM] Fix use of var defined in CHECK-NOT

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.

LLVM test CodeGenPrepare/ARM/sink-add-mul-shufflevector.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. The
bug was then copied over in
Transforms/CodeGenPrepare/ARM/sink-add-mul-shufflevector-inseltpoison.ll

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

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D99597