[RISCV][TableGen] Mark MachineInstr with FrameIndex as not compressible

If a MachineInstr's operand should be Reg in compiler's output but is
currently FrameIndex, `isCompressibleInst()` will terminate at
`MachineOperandType::getReg()`.

This patch adds `.isReg()` checks to make `isCompressibleInst()` return
false for these MachineInstr, allowing `getInstSizeInBytes()` to return
a value and `EstimateFunctionSizeInBytes()` to work as intended.

See https://reviews.llvm.org/D129999#3694222 for details.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D129999