[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

Author: Thomas Preud'homme

llvm/test/CodeGen/Hexagon/hwloop3.ll

@@ -3,8 +3,7 @@
 ; Remove the unconditional jump to following instruction.
 
 ; CHECK: endloop0
-; CHECK-NOT: jump [[L1:.]]
-; CHECK-NOT: [[L1]]
+; CHECK-NOT: jump [[L1:.]]{{.*[[:space:]]+}}[[L1]]
 
 define void @test(i32* nocapture %a, i32 %n) nounwind {
 entry: