x86/sev-es: Split vc_decode_insn()

Split it into two helpers - a user- and a kernel-mode one for
readability. Yes, the original function body is not that convoluted but
splitting it makes following through that code trivial than having to
pay attention to each little difference when in user or in kernel mode.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210304174237.31945-13-bp@alien8.de