[FPEnv][LegalizeTypes] Make ScalarizeVecOp_STRICT_FP_ROUND do its own...

[FPEnv][LegalizeTypes] Make ScalarizeVecOp_STRICT_FP_ROUND do its own replacements and return SDValue()

The caller will assert for nodes with more than 2 results unless
we return a null SDValue.

I tried to test this by copying an AArch64 test for ScalarizeVecOp_FP_ROUND.
While it did hit the assert and this commited fixed that. It also
hit a later problem that couldn't be fixed without adding strict
FP support to AArch64.