Fix ty_half_width in isle prelude

[scottmcm]

I spent way too long trying to figure out how I'd gotten a rule wrong, only to find out that ty_half_width in ISLE was doing something different from Type::half_width in the rest of the codebase.

Thus this PR, which stops it having a custom implementation in ISLE and just calls the normal method. I think anywhere in ISLE rules that wants to exclude the other cases should be using fits_in_64 or similar, not having the ty_half_width extern do something surprising.

The existing uses

wasmtime/cranelift/codegen/src/isa/riscv64/lower.isle

Lines 115 to 123 in 37300d3

	(rule 10 (lower (has_type (ty_vec_fits_in_register ty) (iadd x (splat (sextend y @ (value_type sext_ty))))))
	(if-let half_ty (ty_half_width ty))
	(if-let $true (ty_equal (lane_type half_ty) sext_ty))
	(rv_vwadd_wx x y (unmasked) (vstate_mf2 half_ty)))
	(rule 10 (lower (has_type (ty_vec_fits_in_register ty) (iadd x (splat (uextend y @ (value_type uext_ty))))))
	(if-let half_ty (ty_half_width ty))
	(if-let $true (ty_equal (lane_type half_ty) uext_ty))
	(rv_vwaddu_wx x y (unmasked) (vstate_mf2 half_ty)))

are under ty_vec_fits_in_register already, so it's fine.