ext-dce: Fix subreg_lsb is_constant assumption

ext-dce: Fix subreg_lsb is_constant assumption

ext-dce had:

	  if (SUBREG_P (dst) && SUBREG_BYTE (dst).is_constant ())
	    {
	      bit = subreg_lsb (dst).to_constant ();
	      if (bit >= HOST_BITS_PER_WIDE_INT)
		bit = HOST_BITS_PER_WIDE_INT - 1;
	      dst = SUBREG_REG (dst);

But a constant SUBREG_BYTE doesn't guarantee a constant subreg_lsb.
If the SUBREG_REG is a pair of N-bit registers on a big-endian target,
the most significant end has a SUBREG_BYTE of 0 but a subreg_lsb of N.
This N would then be non-constant for variable-length registers.

The patch fixes gcc.dg/torture/pr120276.c and other failures on
aarch64_be-elf.

gcc/
	* ext-dce.cc (ext_dce_process_uses): Apply is_constant directly
	to the subreg_lsb.

(cherry picked from commit bf3037e923e9f91d93ab64bdf73a37f64f659fb9)