Removed rD and rS1 from TeeRISCInst class and introduced at F1, F2 and F3.

Added EncoderMethod for complex MEM format - "getMemEncoding"

Added rExtra and cond parameters to F1 instruction class. This will enable
instructions to specify these values in future.

Fix OR_IMM to use imm16 instead of rS2

Renamed the following operand/functions to be consistent with other backends.
1) ADDR => iaddr
2) SelectADDR => SelectAddrRegImm
3) MEM => memri

Add mayLoad and mayStore attributes to LD and ST instructions appropriately.
Renamed SelectADDR to SelectAddrRegImm
Added getMemEncoding() for memri types.
Minor code cleanup in DecodeMem()