[clang] [clang][codegen] Fix possible crash when setting TBAA metadata on FP math libcalls (PR #108575)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

I managed to reduce the breakage to the following snippet:
```c
long double powl(long double a, long double b);
long double a() { return powl(2.0L, 2.0L); }
```
Compiled like this:
```
$ clang -target x86_64-w64-mingw32 -S -O2 -o out.s repro.c
```
The output between before and after this change differs like this:
```diff
--- out-good.s  2024-09-16 13:45:09.505125890 +0300
+++ out-bad.s   2024-09-16 13:45:09.533125294 +0300
@@ -10,12 +10,7 @@
        .scl    2;
        .type   32;
        .endef
-       .section        .rdata,"dr"
-       .p2align        2, 0x0                          # -- Begin function a
-.LCPI0_0:
-       .long   0x40000000                      # float 2
-       .text
-       .globl  a
+       .globl  a                               # -- Begin function a
        .p2align        4, 0x90
 a:                                      # @a
 .seh_proc a
@@ -26,16 +21,10 @@
        .seh_stackalloc 80
        .seh_endprologue
        movq    %rcx, %rsi
-       flds    .LCPI0_0(%rip)
-       fld     %st(0)
-       fstpt   48(%rsp)
-       fstpt   32(%rsp)
        leaq    64(%rsp), %rcx
        leaq    48(%rsp), %rdx
        leaq    32(%rsp), %r8
        callq   powl
-       fldt    64(%rsp)
-       fstpt   (%rsi)
        movq    %rsi, %rax
        addq    $80, %rsp
        popq    %rsi
```

I'll push a revert shortly.

https://github.com/llvm/llvm-project/pull/108575