[X86] When using AND+PACKUS in lowerV16I8Shuffle, generate the build vector...

[X86] When using AND+PACKUS in lowerV16I8Shuffle, generate the build vector directly in v16i8 with the correct 0x00 or 0xFF elements rather than using another VT and bitcasting it.

The build_vector will become a constant pool load. By using the
desired type initially, it ensures we don't generate a bitcast
of the constant pool load which will need to be folded with
the load.

While experimenting with another patch, I noticed that when the
load type and the constant pool type don't match, then
SimplifyDemandedBits can't handle it. While we should probably
fix that, this was a simple way to fix the issue I saw.

llvm-svn: 366732