[PR rtl-optimization/116437] Fix RTL checking issue in ext-dce

Another RTL checking failure in ext-dce.  An easy one to fix this time.

When we optimize an extension we have to go back and cleanup with
SUBREG_PROMOTED state.  So we record the destination register into a bitmap as
we make changes, then later do a single pass over the IL fixing any associated
subreg expressions.

The optimization is changing the SET_SRC and largely ignores the destination.
The LHS could be a REG, SUBREG, or ZERO_EXTRACT.  If the LHS is a SUBREG or
ZERO_EXTRACT we can just strip them.

Bootstrapped and ran the testsuite with an RTL checking compiler and verified
no ext-dce RTL checking failures tripped.  Also bootstrapped and regression
tested x86_64 in the usual way.

Pushing to the trunk.

	PR rtl-optimization/116437
gcc/
	* ext-dce.cc (ext_dce_try_optimize_insn): Handle SUBREG and
	ZERO_EXTRACT destinations.