Revert "aco: Combine v_cvt_u32_f32 with insert to v_cvt_pk_u8_f32."

This reverts commit 6d020540.

v_cvt_pk_u8_f32 returns 0xff instead of v_cvt_u32_f32 & 0xff if the input is larger than 255.

Closes: #8128 (closed)

Cc: mesa-stable