[clang] [clang][x86] Fix the return type of the cvtpd2dq builtin (PR #189254)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

Sure - keep each patch focused on a particular instruction type though and expect some issues with constexpr evaluation which often uses the signedness from the types in the APSInt values

https://github.com/llvm/llvm-project/pull/189254