Legalize neg: -x becomes 0 - x (#2128) (#2145)

This fixes an error with negating a negative SInt literal and a
[debatable] lint warning in Verilator when negating any value.

This behavior matches that of Chisel (which directly emits the 0 - x
already).

(cherry picked from commit 49b82324

)

Co-authored-by: Jack Koenig <koenig@sifive.com>