[X86] Make LowerOperationWrapper more robust. Remove now unnecessary...

[X86] Make LowerOperationWrapper more robust. Remove now unnecessary ReplaceAllUsesWith from LowerMSCATTER.

Previously LowerOperationWrapper took the number of results from the original
node and counted that many results from the new node. This was intended to drop
chain operands from FP_TO_SINT lowering that uses X87 with memory operations to
stack temporaries. The final load had an extra chain output that needs to be
ignored.

Unfortunately, it didn't work with scatter which has 2 result operands, the
mask output which is discarded and a chain output. The chain output is the one
that is needed but it comes second and it would be dropped by the previous
logic here. To workaround this we were doing a ReplaceAllUses in the lowering
code so that the generic legalization code wouldn't see any uses to replace
since it had been given the wrong result/type.

After this change we take the LowerOperation result directly if the original
node has one result. This allows us to directly return the chain from scatter
or the load data from the FP_TO_SINT case. When the original node has multiple
results we'll ensure the returned node has the same number and copy them over.
For cases where the original node has multiple results and the new code for some
reason has even more results, MERGE_VALUES can be used to pass only the needed
results.

llvm-svn: 357887