tree-optimization/98221 - fix wrong unpack operation used for big-endian

tree-optimization/98221 - fix wrong unpack operation used for big-endian

The vec-abi-varargs-1.c testcase on IBM Z currently fails.

While adding an SI mode vector to a DI mode vector the first is unpacked using:

  _28 = BIT_INSERT_EXPR <{ 0, 0, 0, 0 }, _2, 0>;
  _34 = [vec_unpack_lo_expr] _28;

However, on big endian targets lo refers to the right hand side of the vector - in this case the zeroes.

2021-01-11  Andreas Krebbel  <krebbel@linux.ibm.com>

	* tree-ssa-forwprop.c (simplify_vector_constructor): For
	big-endian, use UNPACK[_FLOAT]_HI.