[Issue 18461] core.bitop.bt returns garbage in release code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18461

ag0aep6g at gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
          Component|phobos                      |dmd

--- Comment #7 from ag0aep6g at gmail.com ---
Good code generated with `-O -inline` (no `-release`):

----
0000000000000000 <_Dmain>:
   0:   55                      push   rbp
   1:   48 8b ec                mov    rbp,rsp
   4:   48 83 ec 10             sub    rsp,0x10
   8:   48 c7 45 f8 10 00 00    mov    QWORD PTR [rbp-0x8],0x10
   f:   00 
  10:   48 8d 45 f8             lea    rax,[rbp-0x8]
  14:   48 0f ba 20 04          bt     QWORD PTR [rax],0x4
---- 

Bad code with `-release -O -inline`:

----
0000000000000000 <_Dmain>:
   0:   55                      push   rbp
   1:   48 8b ec                mov    rbp,rsp
   4:   48 83 ec 10             sub    rsp,0x10
   8:   48 8d 45 f8             lea    rax,[rbp-0x8]
   c:   48 0f ba 20 04          bt     QWORD PTR [rax],0x4
----

Note that `mov QWORD PTR [rbp-0x8],0x10` is missing in the bad code. I.e.,
`test_val` doesn't get initialized.

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