[AArch64] Assembler support for the ARMv8.2a dot product instructions

Dot product is an optional ARMv8.2a extension, see also the public architecture
specification here:
https://developer.arm.com/products/architecture/a-profile/exploration-tools.
This patch adds AArch64 assembler support for these dot product instructions.

Differential Revision: https://reviews.llvm.org/D36515

llvm-svn: 310480
diff --git a/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp b/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
index 4173765..03caaf0 100644
--- a/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ b/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -1810,6 +1810,8 @@
       .Case(".d", true)
       // Needed for fp16 scalar pairwise reductions
       .Case(".2h", true)
+      // another special case for the ARMv8.2a dot product operand
+      .Case(".4b", true)
       .Default(false);
 }