X86AsmParser AVX-512: Return error instead of hitting assert

When parsing a sequence of tokens beginning with {, it will hit an assert and crash if the token afterwards is not an identifier. Instead of this, return a more verbose error as seen elsewhere in the function.

Patch by Brandon Jones (BrandonTJones)

Differential Revision: https://reviews.llvm.org/D57375

llvm-svn: 354356