Extended Description

$ cat >bad.ll <<EOL
define i32 @​src(i32 %x1, i32 %x2) {
%x14 = add i32 %x1, %x2
%x6 = icmp ugt i32 %x14, %x2
%x9 = select i1 %x6, i32 %x14, i32 -1
ret i32 %x9
}
EOL

$ opt bad.ll --instcombine -S
; ModuleID = 'bad.ll'
source_filename = "bad.ll"

define i32 @​src(i32 %x1, i32 %x2) {
%1 = call i32 @​llvm.uadd.sat.i32(i32 %x2, i32 %x1)
ret i32 %1
}

; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
declare i32 @​llvm.uadd.sat.i32(i32, i32) #​0

attributes #​0 = { nofree nosync nounwind readnone speculatable willreturn }

Alive output:

define i32 @​src(i32 %x1, i32 %x2) {
%0:
%x14 = add i32 %x1, %x2
%x6 = icmp ugt i32 %x14, %x2
%x9 = select i1 %x6, i32 %x14, i32 4294967295
ret i32 %x9
}
=>
define i32 @​tgt(i32 %x1, i32 %x2) {
%0:
%1 = uadd_sat i32 %x2, %x1
ret i32 %1
}
Transformation doesn't verify!
ERROR: Value mismatch

Example:
i32 %x1 = #x00000000 (0)
i32 %x2 = #x04000000 (67108864)

Source:
i32 %x14 = #x04000000 (67108864)
i1 %x6 = #x0 (0)
i32 %x9 = #xffffffff (4294967295, -1)

Target:
i32 %1 = #x04000000 (67108864)
Source value: #xffffffff (4294967295, -1)
Target value: #x04000000 (67108864)