match.pd: Fix sve/vcond_3.c

The sve/vcond_3.c tests had started to fail after PR97690/99305,
because we were generating:

  a = VEC_COND_EXPR <cond, { 0, ... }, { 1, ... }>;
  b = a << 15;

instead of:

  a = VEC_COND_EXPR <cond, { 0, ... }, { 1<<15, ... }>;

We already have a match.pd rule to handle this kind of thing,
but it didn't handle shifts.

gcc/
	* match.pd: Extend vec_cond folds to handle shifts.