[InstCombine] Sink sub into hands of select if one hand becomes zero. Part 2 (PR44426)

This decreases use count of %Op0, makes one hand of select to be 0,
and possibly exposes further folding potential.

Name: sub %Op0, (select %Cond, %Op0, %FalseVal) -> select %Cond, 0, (sub %Op0, %FalseVal)
  %Op0 = %TrueVal
  %o = select i1 %Cond, i8 %Op0, i8 %FalseVal
  %r = sub i8 %Op0, %o
=>
  %n = sub i8 %Op0, %FalseVal
  %r = select i1 %Cond, i8 0, i8 %n

Name: sub %Op0, (select %Cond, %TrueVal, %Op0) -> select %Cond, (sub %Op0, %TrueVal), 0
  %Op0 = %FalseVal
  %o = select i1 %Cond, i8 %TrueVal, i8 %Op0
  %r = sub i8 %Op0, %o
=>
  %n = sub i8 %Op0, %TrueVal
  %r = select i1 %Cond, i8 %n, i8 0

https://rise4fun.com/Alive/aHRt

https://bugs.llvm.org/show_bug.cgi?id=44426