Fix PR19144: Incorrect offset generated for int-to-fp conversion at -O0.

When converting a signed 32-bit integer to double-precision floating point on
hardware without a lfiwax instruction, we have to instead use a lfd followed
by fcfid.  We were erroneously offsetting the address by 4 bytes in
preparation for either a lfiwax or lfiwzx when generating the lfd.  This fixes
that silly error.

This was not caught in the test suite since the conversion tests were run with
-mcpu=pwr7, which implies availability of lfiwax.  I've added another test
case for older hardware that checks the code we expect in the absence of
lfiwax and other flavors of fcfid.  There are fewer tests in this test case
because we punt to DAG selection in more cases on older hardware.  (We must
generate complex fiddly sequences in those cases, and there is marginal
benefit in duplicating that logic in fast-isel.)