[InstCombine][NFC] Tests for 'unsigned sub overflow' check

[InstCombine][NFC] Tests for 'unsigned sub overflow' check

----------------------------------------
Name: unsigned sub, overflow, v0
  %sub = sub i8 %x, %y
  %ov = icmp ugt i8 %sub, %x
=>
  %agg = usub_overflow i8 %x, %y
  %sub = extractvalue {i8, i1} %agg, 0
  %ov = extractvalue {i8, i1} %agg, 1

Done: 1
Optimization is correct!

----------------------------------------
Name: unsigned sub, no overflow, v0
  %sub = sub i8 %x, %y
  %ov = icmp ule i8 %sub, %x
=>
  %agg = usub_overflow i8 %x, %y
  %sub = extractvalue {i8, i1} %agg, 0
  %not.ov = extractvalue {i8, i1} %agg, 1
  %ov = xor %not.ov, -1

Done: 1
Optimization is correct!

llvm-svn: 371099