genmodes: Update GET_MODE_MASK when changing NUNITS [PR98214]

genmodes: Update GET_MODE_MASK when changing NUNITS [PR98214]

The static GET_MODE_MASKs for SVE vectors are based on the
static precisions, which in turn are based on 128-bit SVE.
The precisions are later updated based on -msve-vector-bits
(usually to become variable length), but the GET_MODE_MASK
stayed the same.  This caused combine to fold:

  (*_extract:DI (subreg:DI (reg:VNxMM R) 0) ...)

to zero because the extracted bits appeared to be insignificant.

gcc/
	PR rtl-optimization/98214
	* genmodes.c (emit_insn_modes_h): Emit a definition of CONST_MODE_MASK.
	(emit_mode_mask): Treat mode_mask_array as non-constant if adj_nunits.
	(emit_mode_adjustments): Update GET_MODE_MASK when updating
	GET_MODE_NUNITS.
	* machmode.h (mode_mask_array): Use CONST_MODE_MASK.

(cherry picked from commit 0411210fddbd3ec27c8dc1183f40f662712a2232)