Convert constants added by v128.const to the appropriate type before use
Convert constants added by v128.const to the appropriate type before use

By default, constants added by SIMD's v128.const will be typed as I8x16 in CLIF. This type must be changed to the appropriate vector type before use to satisfy cranelift's type checking. To do this, we track what SSA values are created by v128.const and convert them with a raw_bitcast immediately before use in the currently implemented SIMD instructions.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne