dce: Remove __builtin_stack_save during dce [PR122037]

dce: Remove __builtin_stack_save during dce [PR122037]

__builtin_stack_save can be removed when the lhs becomes unused
as it is just recording the current StackPointer into another register.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/122037

gcc/ChangeLog:

	* tree-ssa-dce.cc (eliminate_unnecessary_stmts): Remove
	__builtin_stack_save when the lhs is unused.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vla-1.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>