[Hexagon, test] Fix use of undef FileCheck var

LLVM test CodeGen/Hexagon/hwloop3.ll tries to check for the absence of a
sequence of consecutive 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 merges the two CHECK-NOT into a single CHECK-NOT that
matches the content of two successive non-blank lines, thereby allowing
to preserve the intent of the test.

Reviewed By: bcahoon

Differential Revision: https://reviews.llvm.org/D99778