[x86] Fix infinite loop inside DAG combiner with lzcnt feature.

The issue affects targets supporting fast-lzcnt such as btver2.
This removes extraneous zext/trunc node insertions to fix the infinite
loop.
This fixes Issue https://github.com/llvm/llvm-project/issues/54694

Differential Revision: https://reviews.llvm.org/D122900

Reviewed By: RKSimon, spatel, lebedev.ri

(cherry picked from commit a3d5f1cf

)
Signed-off-by: Warren Ristow <warren.ristow@sony.com>

In https://reviews.llvm.org/D122900 a new function (to exercise the
infinite-loop bug) was added to llvm/test/CodeGen/X86/lzcnt-zext-cmp.ll.
In applying the fix in the main branch, two previously existing
functions in that test also changed behavior slightly, and in the review
it was noted:
    The instructions generated end up being reordered in some cases
    but I think it is equivalent.
That reordering did not happen in those pre-existing functions when
applying the fix to the slightly older code-base of the llvm14 branch,
and so they are suppressed here.  So the updated version of the test in
this commit has the additional function added to it, but it is otherwise
identical to the previous llvm14 version of the test.