[clang][x86] Fix the return type of the cvtpd2dq builtin (#189254)
The CVTPD2DQ instruction converts packed 64-bit floating-point values to
packed 32-bit signed integer values. This patch fixes the return type of
the corresponding builtin, which previously returned a vector of two
64-bit signed integers. The new behavior is in line with the return type
of the CVTTPD2DQ builtin.