x86/insn: Rename insn_decode() to insn_decode_from_regs()

Rename insn_decode() to insn_decode_from_regs() to denote that it
receives regs as param and uses registers from there during decoding.
Free the former name for a more generic version of the function.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210304174237.31945-2-bp@alien8.de