Revert "STFBranchReader: Always initialize REG_0 in src/dest operand state maps"

This reverts commit 07f808cdb4bd93394c4655019cd7063fd77e7b97.

The original patch fixed a bug (checking RS1 twice instead of checking
RS1 and RS2), but broke detection of JAL/JR instructions since it added
superfluous X0 src/dest operands. This reverts the patch and fixes the
original RS1/RS2 bug.