[InstCombine] shoudn't set exact on shr (shl ...) for undefined inputs

[bongjunj]

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

Lines 991 to 995 in 6bf214b

	// shr (shl X, Y), Y
	if (match(I.getOperand(0), m_Shl(m_Value(), m_Specific(I.getOperand(1))))) {
	I.setIsExact();
	return true;
	}

Alive2 report: https://alive2.llvm.org/ce/z/q2vHmJ

----------------------------------------
define i32 @t6_twoshifts.2(i32 %x, i8 %shamt) {
bb:
  %#0 = ctpop i8 %shamt
  %shamt_wide = sext i8 %#0 to i32
  br label %work

work:
  %n0 = shl i32 %x, %shamt_wide
  %r = ashr i32 %n0, %shamt_wide
  br label %end

end:
  ret i32 %r
}
=>
define i32 @t6_twoshifts.2(i32 %x, i8 %shamt) {
bb:
  br label %work

work:
  br label %end

end:
  %#0 = ctpop i8 %shamt
  %#range_0_%#0 = !range i8 %#0, i8 0, i8 9
  %shamt_wide = zext nneg i8 %#range_0_%#0 to i32
  %n0 = shl i32 %x, %shamt_wide
  %r = ashr exact i32 %n0, %shamt_wide
  ret i32 %r
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
i32 %x = #x00000002 (2)
i8 %shamt = undef

Source:
i8 %#0 = #x00 (0)	[based on undef value]
i32 %shamt_wide = #x00000000 (0)
  >> Jump to %work
i32 %n0 = #x00000002 (2)
i32 %r = #x00000002 (2)	[based on undef value]
  >> Jump to %end

Target:
  >> Jump to %work
  >> Jump to %end
i8 %#0 = #x00 (0)
i8 %#range_0_%#0 = #x00 (0)
i32 %shamt_wide = #x00000000 (0)
i32 %n0 = #x00000002 (2)
i32 %r = poison
Source value: #x00000002 (2)
Target value: poison

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors