[RISCV] Return false for unsupported VTs in isFPImmLegal.

I don't have a test case that fails for this, but it seemed like
we should only handle legal types. The callers I looked at in
DAGCombine either check the type is legal or don't even call
isFPImmLegal unless LegalOperations is true.

Written in a slightly odd way because switches on EVT require
an additional isSimple check so an if/else chain is easier. Used a bool
to shorten the code instead of having multiple ifs and returns.
AArch64 uses a similarish structure.

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