Strip of a vector load which is only used partially.

Strip of a vector load which is only used partially.

Optimize

  _4 = VEC_PERM_EXPR <_1, _1, { 4, 5, 6, 7, 4, 5, 6, 7 }>;
  _5 = BIT_FIELD_REF <_4, 128, 0>;

to

  _5 = BIT_FIELD_REF <_1, 128, 128>;

gcc/ChangeLog:

	PR tree-optimization/102583
	* tree-ssa-forwprop.cc (simplify_bitfield_ref): Extended to a
	contiguous stride in the VEC_PERM_EXPR.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr102583.c: New test.
	* gcc.target/i386/pr92645-2.c: Adjust testcase.
	* gcc.target/i386/pr92645-3.c: Ditto.