[SystemZ] Add some generic (floating point support) load instructions.

Add generic instructions for load complement, load negative and load positive
for fp32 and fp64, and let isel prefer them. They do not clobber CC, and so
give scheduler more freedom. SystemZElimCompare pass will convert them when it
can to the CC-setting variants.

Regression tests updated to expect the new opcodes in places where the old ones
where used. New test case SystemZ/fp-cmp-05.ll checks that
SystemZCompareElim.cpp can handle the new opcodes.

README.txt updated (bullet removed).

Note that fp128 is not yet handled, because it is relatively rare, and is a
bit trickier, because of the fact that l.dfr would operate on the sign bit of
one of the subregisters of a fp128, but we would not want to copy the other
sub-reg in case src and dst regs are not the same.

Reviewed by Ulrich Weigand.

llvm-svn: 249046