In order to prevent simplification of a COND_OP with degenerate mask
(CONSTM1_RTX) into just an OP in the presence of length masking this
patch introduces a length-masked analog to VEC_COND_EXPR:
IFN_VCOND_MASK_LEN.
It also adds new match patterns that allow the combination of
unconditional unary, binary and ternay operations with the
VCOND_MASK_LEN into a conditional operation if the target supports it.