Suboptimal/inconsistent codegen — clang doesn't realize an instruction save unless a while loop is manually transformed

Given the following simple C++ function, which counts the number of holes in the numbers in the string: (0,4,6,9 have 1 hole, 8 has two holes) (the same code as #160710):
```cpp
#include
#include
using ui = std::uint_fast32_t;
using uc = std::uint_fast8_t;
ui countholes(const char* s){
constexpr static std::array pre_table{1,0,0,0,1,0,1,0,2,1};
uc c;
ui tot = 0;
while(true){
c = uc(*s++);
if(c
#include
using ui = std::uint_fast32_t;
using uc = std::uint_fast8_t;
extern "C" ui countholes(const char* s){
constexpr static std::array pre_table{1,0,0,0,1,0,1,0,2,1};
uc c;
ui tot = 0;
c = uc(*s++);
if(c=uc('0'));
return tot;
}
```
The assembly output is:
```asm
; SNIP
.LBB0_4:
movzx ecx, cl
add rax, qword ptr [rdx + 8*rcx - 384]
movzx ecx, byte ptr [rdi]
inc rdi
cmp cl, 47
ja .LBB0_4
ret
```
Notice how the `-384` displacement is now added onto the effective address, and the add to `ecx` is gone. For some reason, the compiler doesn't realize the optimization unless the loop is inverted.

No source file or test is named. Start by compiling the provided C++ reproducer and comparing the two Godbolt assembly outputs, then trace the relevant loop and addressing transformations in LLVM's compiler pipeline. Done means identifying the missed code-generation opportunity and adding a regression test that distinguishes the original loop form from the manually inverted form.

Written by the indexing model from the issue text.