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

LLVM test Transforms/LoopVectorize/X86/x86-pr39099.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 only checks for the absence of a widened load which rules
out the presence of the whole sequence and does not involve an undefined
variable.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D99583