vect: Fix vectorized BIT_FIELD_REF for signed bit-fields [PR110557]

If a bit-field is signed and it's wider than the output type, we must
ensure the extracted result sign-extended.  But this was not handled
correctly.

For example:

    int x : 8;
    long y : 55;
    bool z : 1;

The vectorized extraction of y was:

    vect__ifc__49.29_110 =
      MEM <vector(2) long unsigned int> [(struct Item *)vectp_a.27_108];
    vect_patt_38.30_112 =
      vect__ifc__49.29_110 & { 9223372036854775552, 9223372036854775552 };
    vect_patt_39.31_113 = vect_patt_38.30_112 >> 8;
    vect_patt_40.32_114 =
      VIEW_CONVERT_EXPR<vector(2) long int>(vect_patt_39.31_113);

This is obviously incorrect.  This pach has implemented it as:

    vect__ifc__25.16_62 =
      MEM <vector(2) long unsigned int> [(struct Item *)vectp_a.14_60];
    vect_patt_31.17_63 =
      VIEW_CONVERT_EXPR<vector(2) long int>(vect__ifc__25.16_62);
    vect_patt_32.18_64 = vect_patt_31.17_63 << 1;
    vect_patt_33.19_65 = vect_patt_32.18_64 >> 9;

gcc/ChangeLog:

	PR tree-optimization/110557
	* tree-vect-patterns.cc (vect_recog_bitfield_ref_pattern):
	Ensure the output sign-extended if necessary.

gcc/testsuite/ChangeLog:

	PR tree-optimization/110557
	* g++.dg/vect/pr110557.cc: New test.

(cherry picked from commit 63ae6bc6)