C-- pipeline/NCG fails to optimize simple repeated addition

While debugging #14346 (closed) I noticed some rather abhorrent code in a disassembly of the newPinnedByteArray# primop:

Dump of assembler code for function stg_newPinnedByteArrayzh:
   0x00000000004a8518 <+0>:	mov    0x378(%r13),%rax
   0x00000000004a851f <+7>:	cmpq   $0x0,0x10(%rax)
   0x00000000004a8524 <+12>:	je     0x4a8593 <stg_newPinnedByteArrayzh+123>
   0x00000000004a8526 <+14>:	mov    0x4f5730,%rax
   0x00000000004a852e <+22>:	mov    0x38(%rax),%rax
   0x00000000004a8532 <+26>:	cmp    0x4f5718,%rax
   0x00000000004a853a <+34>:	jae    0x4a8593 <stg_newPinnedByteArrayzh+123>
   0x00000000004a853c <+36>:	mov    %rbx,%rax
   0x00000000004a853f <+39>:	lea    0x7(%rax),%rcx
   0x00000000004a8543 <+43>:	shr    $0x3,%rcx
   0x00000000004a8547 <+47>:	add    $0x10,%rax       <--- starts here
   0x00000000004a854b <+51>:	add    $0xf,%rax
   0x00000000004a854f <+55>:	add    $0x7,%rax
   0x00000000004a8553 <+59>:	shr    $0x3,%rax
   0x00000000004a8557 <+63>:	mov    $0x49d820,%ecx
   ...

That is three successive add instructions; surely those should be collapsed into one by the Cmm-to-Cmm pipeline.