Split have_instr definition out of always block in if_stage
Split have_instr definition out of always block in if_stage

This should cause no functional change, but avoids a seeming
combinatorial loop reported by Verilator.

The seeming loop is because the always_comb process that contained
have_instr is sensitive to if_id_pipe_reg_we but that process wrote
the have_instr signal, which is used in the continuous assignment to
if_id_pipe_reg_we later on.

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