Look like this is a case where we miss hoisting the common operand from the multiplications when shl is used instead of mul: https://godbolt.org/z/66cqMK

rewriting as:

int foo2(int p)
{
unsigned i, s = 0;
if (p) {
for (i = 4; i < 20; i++)
s += i2; // works for i3
} else {
for (i = 4; i < 20; i++)
s += i4; // works for i3
}
return s;
}

gives:

_Z4foo2i:
testl %edi, %edi
movl $736, %ecx # imm = 0x2E0
movl $368, %eax # imm = 0x170
cmovel %ecx, %eax
retq

Interesting, so failed loop unswitch for original test case?

Look like this is a case where we miss hoisting the common operand from the
multiplications when shl is used instead of mul:
https://godbolt.org/z/66cqMK

Instcombine is definitely missing a factorization for shl with add/sub. I was looking at that fold as part of solving bug 47430.

I added some tests here:
1dd4c4e

Let me see if adding that fixes the example in the description.

With the instcombine improvement, the original example collapses to the optimal code:
7903ae4