[X86] Limit vpermil2pd/vpermil2ps immediates to 4 bits in the assembly parser.

[X86] Limit vpermil2pd/vpermil2ps immediates to 4 bits in the assembly parser.

The upper 4 bits of the immediate byte are used to encode a
register. We need to limit the explicit immediate to fit in the
remaining 4 bits.

Fixes PR42899.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368123 91177308-0d34-0410-b5e6-96231b3b80d8