Avoid duplicate vector initializations during RTL expansion.

This middle-end patch avoids some redundant RTL for vector initialization
during RTL expansion.  For the simple test case:

typedef __int128 v1ti __attribute__ ((__vector_size__ (16)));
__int128 key;

v1ti foo() {
    return (v1ti){key};
}

the middle-end currently expands:

(set (reg:V1TI 85) (const_vector:V1TI [ (const_int 0) ]))

(set (reg:V1TI 85) (mem/c:V1TI (symbol_ref:DI ("key"))))

where we create a dead instruction that initializes the vector to zero,
immediately followed by a set of the entire vector.  This patch skips
this zeroing instruction when the vector has only a single element.
It also updates the code to indicate when we've cleared the vector,
so that we don't need to initialize zero elements.

2023-06-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expr.cc (store_constructor) <case VECTOR_TYPE>: Don't bother
	clearing vectors with only a single element.  Set CLEARED if the
	vector was initialized to zero.