Add Swizzle() intrinsics for Byte16, Byte8, and Byte4, and add Byte4
constructors and assignment operators. Also move LLVM-specific
implementations to the generic Reactor.cpp source file.

On x86 these all translate to a pshufb instruction, which is very
efficient.

Bug: b/148295813
Change-Id: Icf88fe1621623f8104c4a642d560643a01b9ef55
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40549
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>