`mul nuw`+`lshr exact` should fold to a single multiplication (when the latter is a factor)

[scottmcm]

Alive for the specific example, with opt+llc: https://alive2.llvm.org/ce/z/7oofsh
+label: llvm:optimizations

I tried the following:

define i64 @src(i64 noundef %0) {
start:
  %1 = mul nuw nsw i64 %0, 52
  %2 = lshr exact i64 %1, 2
  ret i64 %2
}

Since 52 = 4 * 13 I expected to see that fold down to just a single mul:

define i64 @tgt(i64 noundef %0) {
start:
  %1 = mul nuw nsw i64 %0, 13
  ret i64 %1
}

But it doesn't -- the mul and lshr are left in the optimized IR, and aren't folded by the target either:

src:                                    # @src
        imul    rax, rdi, 52
        shr     rax, 2
        ret

Whereas the single-multiplication version ends up not even needing an imul:

tgt:                                    # @tgt
        lea     rax, [rdi + 2*rdi]
        lea     rax, [rdi + 4*rax]
        ret

So in general, %1 = mul nuw %0, CONST1; %2 = lshr exact %1, CONST2 should simplify to a single mul nuw whenever CONST1 is a multiple of 1 << CONST2.

In fact, it looks like this already happens in opt if written with div: https://alive2.llvm.org/ce/z/L-VouQ

  %1 = mul nuw i64 %0, 52
  %2 = udiv exact i64 %1, 4
=>
  %1 = mul nuw nsw i64 %0, 13

So that code path just needs to take into account places where the udiv -> lshr transformation had already happened.

[bcl5980]

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

Line 1171 in e4903d8

if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&

I think we can add the combination here.

[bcl5980]

I'm trying to fix the issue on https://reviews.llvm.org/D123453