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

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

This decreases use count of %Op1, makes one hand of select to be 0,
and possibly exposes further folding potential.

Name: sub (select %Cond, %Op1, %FalseVal), %Op1 -> select %Cond, 0, (sub %FalseVal, %Op1)
  %Op1 = %TrueVal
  %o = select i1 %Cond, i8 %Op1, i8 %FalseVal
  %r = sub i8 %o, %Op1
=>
  %n = sub i8 %FalseVal, %Op1
  %r = select i1 %Cond, i8 0, i8 %n

Name: sub (select %Cond, %TrueVal, %Op1), %Op1 -> select %Cond, (sub %TrueVal, %Op1), 0
  %Op1 = %FalseVal
  %o = select i1 %Cond, i8 %TrueVal, i8 %Op1
  %r = sub i8 %o, %Op1
=>
  %n = sub i8 %TrueVal, %Op1
  %r = select i1 %Cond, i8 %n, i8 0

https://rise4fun.com/Alive/avL

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