[BPF] Mark pseudo insn patterns as isCodeGenOnly

These pseudos are not supposed to be visible to user.

This patch reduced the auto-generated instruction matcher. For example,
the following words are removed from keyword list of LLVM BPF assembler.

-  MCK__35_, // '#'
-  MCK__COLON_, // ':'
-  MCK__63_, // '?'
-  MCK_ADJCALLSTACKDOWN, // 'ADJCALLSTACKDOWN'
-  MCK_ADJCALLSTACKUP, // 'ADJCALLSTACKUP'
-  MCK_PSEUDO, // 'PSEUDO'
-  MCK_Select, // 'Select'

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
llvm-svn: 322535