fpnew: Fix f2i cast edge cases

Created by: huettern

Fixes an edge-case where an overflow before rounding causes the integer output to overflow/wrap instead of clip.

The bug was found when converting the float 127.5 to an INT8 with RNE rounding. 127.5 is converted to 128 before rounding which resulted in the output 0x80 instead of 0x7f.

Original fix by @stmach. Reviewed-by: Noah Huetter noah.huetter@axelera.ai