[PR42093] [arm] [thumb2] disable tree-dce for test

[PR42093] [arm] [thumb2] disable tree-dce for test

CD-DCE introduces blocks to share common PHI nodes, which replaces a
backwards branch that used to prevent the thumb2 jump table shortening
that PR42093 tested for.  In order to keep on testing that the
backward branch prevents the jumptable shortening, disable tree-dce.

for  gcc/testsuite/ChangeLog

	PR target/42093
	* gcc.target/arm/pr42093.c: Disable tree-dce.