expr: Fix up VEC_PACK_TRUNC_EXPR expansion [PR101046]

expr: Fix up VEC_PACK_TRUNC_EXPR expansion [PR101046]

The following testcase ICEs, because we have a mode mismatch.
VEC_PACK_TRUNC_EXPR's operands have different modes from the result
(same vector mode size but twice as large element),
but we were passing non-NULL subtarget with the mode of the result
to the expansion of its arguments, so the VEC_PERM_EXPR in one of the
operands which had V8SImode operands and result had V16HImode target.

Fixed by clearing the subtarget if we are changing mode.

2021-06-15  Jakub Jelinek  <jakub@redhat.com>

	PR target/101046
	* expr.c (expand_expr_real_2) <case VEC_PACK_FIX_TRUNC_EXPR,
	case VEC_PACK_TRUNC_EXPR>: Clear subtarget when changing mode.

	* gcc.target/i386/pr101046.c: New test.