Unused `mem_fp` registers/latches with Zfinx enabled

Created by: andreaskurth

With the Zfinx extension enabled, the mem_fp registers are not read, but there currently is an always_ff process even if Zfinx == 1.

Related to this, accesses to we_b_dec and we_a_dec are currently out-of-bounds if Zfinx == 1. This is because we_a_dec and we_b_dec are logic[NUM_TOT_WORDS-1:0], and NUM_TOT_WORDS == NUM_WORDS for FPU and Zfinx enabled.

The same applies to the latch-based register file.