llvm19: backport a fix for const analysis crash on fp128

[hosaka]

Testing the changes

• I tested the changes in this PR: briefly (the repro below compiles with this patch)

Local build testing

• I built this PR locally for my native architecture, (x86_64-glibc)
• I built this PR locally for these architectures (if supported. mark crossbuilds):
  • aarch64-musl (cross)
  • aarch64-glibc (cross)

Summary

This fix was not backported to LLVM 19 and only landed in 20.1.0-rc1: llvm/llvm-project@83a5c7c

A simple repro that triggers this bug, compiled with clang-19 -O1 -c repro.c:

__float128 f128_abs(__float128 x) { return __builtin_fabsf128(x); }

__float128 caller(void) { return f128_abs(0); }

cc @Calandracas606

[skip ci]

[hosaka]

Apologies for messing up the order in [ci skip]...