[PPC64] VSX indexed-form loads use wrong instruction format

The VSX instruction definitions for lxsdx, lxvd2x, lxvdsx, and lxvw4x
incorrectly use the XForm_1 instruction format, rather than the
XX1Form instruction format.  This is likely a pasto when creating
these instructions, which were based on lvx and so forth.  This patch
uses the correct format.

The existing reformatting test (test/MC/PowerPC/vsx.s) missed this
because the two formats differ only in that XX1Form has an extension
to the target register field in bit 31.  The tests for these
instructions used a target register of 7, so the default of 0 in bit
31 for XForm_1 didn't expose a problem.  For register numbers 32-63
this would be noticeable.  I've changed the test to use higher
register numbers to verify my change is effective.