AMDGPU: Stop assuming vreg for build_vector

This was causing a variety of test failures when v2i64
is added as a legal type.

SIFixSGPRCopies should correctly handle the case of vector inputs
to a scalar reg_sequence, so this isn't necessary anymore. This
was hiding some deficiencies in how reg_sequence is handled later,
but this shouldn't be a problem anymore since the register class
copy of a reg_sequence is now done before the reg_sequence.

llvm-svn: 251860