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

Commit 22ce5eb0, changed checks in
GVN/big-endian.ll into CHECK-NOT. The intent was to check that a
succession of lines does not occur but each CHECK-NOT is checked
independently. In other word, one CHECK-NOT uses a variable defined
in a pattern (the one defining the variable) that should not occur in
the input. The bug was then copied over in NewGVN/big-endian.ll.

This commit only checks for the absence of i16 load which rules out the
presence of the whole sequence and does not involve an undefined
variable.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D99581