[RISCV] Replace most uses of RISCVSubtarget::hasStdExtV. NFCI

Add new hasVInstructions() which is currently equivalent.

Replace vector uses of hasStdExtZfh/F/D with new vector specific
versions. The vector spec no longer requires that the vectors implement the
same types as scalar. It only requires that the scalar type is
the maximum size the vectors can support. This is currently
implemented using the scalar rule we were using before.

Add new hasVInstructionsI64() begin using to qualify code that
requires i64 vector elements.

This is all NFC for now, but we can start using this to better
implement D112408 which introduces the Zve extensions.

Reviewed By: frasercrmck, eopXD

Differential Revision: https://reviews.llvm.org/D112496