[RISCV] Be more explicit string replacements in RISCVInstrInfoVPseudos.td. NFC

Instead of replacing "F16", "F32", "F64" anywhere in the string.
Only replace them when proceeded by a "_", "_V", or "_W".

This allows F16/F32/F64 to appear in the instruction name.
This is needed for some vendor instructions in our downstream. We
may be upstreaming them in the future.