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

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Name: unsigned add, overflow, v0
  %add = add i8 %x, %y
  %ov = icmp ult i8 %add, %x
=>
  %agg = uadd_overflow i8 %x, %y
  %add = extractvalue {i8, i1} %agg, 0
  %ov = extractvalue {i8, i1} %agg, 1

Done: 1
Optimization is correct!

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Name: unsigned add, overflow, v1
  %add = add i8 %x, %y
  %ov = icmp ult i8 %add, %y
=>
  %agg = uadd_overflow i8 %x, %y
  %add = extractvalue {i8, i1} %agg, 0
  %ov = extractvalue {i8, i1} %agg, 1

Done: 1
Optimization is correct!

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

Done: 1
Optimization is correct!

----------------------------------------
Name: unsigned add, no overflow, v1
  %add = add i8 %x, %y
  %ov = icmp uge i8 %add, %y
=>
  %agg = uadd_overflow i8 %x, %y
  %add = extractvalue {i8, i1} %agg, 0
  %not.ov = extractvalue {i8, i1} %agg, 1
  %ov = xor %not.ov, -1

Done: 1
Optimization is correct!

llvm-svn: 371098