[InstCombine] narrow truncated add/sub/mul with constant

Name: narrow_sub
  %sub = sub i32 C1, %x
  %r = trunc i32 %sub to i8
  =>  
  %xn = trunc i32 %x to i8
  %narrowC = trunc i32 C1 to i8
  %r = sub i8 %narrowC, %xn
 
Name: narrow_add
  %add = add i32 %x, C1
  %r = trunc i32 %add to i8
  =>  
  %xn = trunc i32 %x to i8
  %narrowC = trunc i32 C1 to i8
  %r = add i8 %xn, %narrowC
  
Name: narrow_mul
  %mul = mul i32 %x, C1
  %r = trunc i32 %mul to i8
  =>  
  %xn = trunc i32 %x to i8
  %narrowC = trunc i32 C1 to i8
  %r = mul i8 %xn, %narrowC


http://rise4fun.com/Alive/QpS

This doesn't solve PR34046 (failure to recognize rotate):
https://bugs.llvm.org/show_bug.cgi?id=34046
...but it reduces an extra complication in the description examples 
to a form that we can more easily match.

llvm-svn: 310141