[rtl] Branch signal timing fix
[rtl] Branch signal timing fix

- Before this fix, the branch signal was qualified by the illegal
  instruction signal and the illegal csr signal.
- This patch removes both of these since the decoder already masks
  branches with illegal isntruction, and a branch cannot be a CSR op.
- This improves the worst path in the design significantly without the
  branch target ALU.

Signed-off-by: Tom Roberts <f3b1a53ee1cbeb35a47033dea932030093853292@lowrisc.org>

AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,AmbiML/ibex