[llvm-diff] False alarm on PHI node of identical modules

[Nuullll]

Reproducer:

$ llvm-diff a.ll a.ll

in function foo:
  in block %exit:
    >   %res = phi double [ 0.000000e+00, %entry ], [ %0, %else ]
    <   %res = phi double [ 0.000000e+00, %entry ], [ %0, %else ]

; a.ll
define double @foo() {
entry:
  br i1 true, label %else, label %exit

else:
  %0 = extractelement <2 x double> zeroinitializer, i64 0
  br label %exit

exit:
  %res = phi double [ 0.000000e+00, %entry ], [ %0, %else ]
  ret double 0.000000e+00
}

llvm version: head c5990d3

[Nuullll]

Working on this: https://reviews.llvm.org/D128024