[RISCV][NFC] Push riscv_selectcc PatFrag Inwards (#175902)

`riscv_selectcc_frag` was a PatFrag with a transform to get a
`RISCVCC::CondCode` from the `riscv_selectcc` node. The pattern matched
the entire `riscv_selectcc` node, but the transform meant that the saved
value was just the condition code itself, which was not obvious (to me),
either from naming or from how the pattern was structured.

This change "pushes" the pattern fragment inwards to the condition code
it relates to using `riscv_cond:$cc` (a `cond` fragment with a
transform, only needed when defining the pattern directly with the
pseudo), or using the `CCtoRISCVCC` transform explicitly in the emit
side of the pattern.

IMO this makes the select handling quite a bit clearer, as the `$cc` in
the pattern and result more directly relates to the operand it matches
and represents. This has potentially swapped one source of confusion
noted above for the confusion of seeing `Cond:$cc` being used to bind
the specific condition code that has been provided at pattern
instantiation to `$cc`, so that it can be transformed before being
emitted.