This commit optimizes instruction fusion detection logic for fused
addw{byte, bit, zexth, sexth}, mulw7, and logic{lsb, zexth}
instructions.

Previously we use fuType and fuOpType from the normal decoder, and this
incurs a bad timing. Now we change the detection logic to use only the
raw instructions. Though the fused instruction still uses the
fuOpType from the normal decoder, there should be only serveral MUXes
left.