[InstCombine] Fold (mul(abs(x),abs(x))) -> (mul(x,x)) (PR39476)

This patch adds support for discarding integer absolutes (abs + nabs variants) from self-multiplications.

ABS Alive2: http://volta.cs.utah.edu:8080/z/rwcc8W
NABS Alive2: http://volta.cs.utah.edu:8080/z/jZXUwQ

This is an InstCombine version of D79304 - I'm not sure yet if we'll need that after this.

Reviewed By: @lebedev.ri and @xbolva00

Differential Revision: https://reviews.llvm.org/D79319