Remove astral character assert in U16MatchCharICase32Insn

Summary:
We currently have an assert that characters encountered in a
`U16MatchCharICase32Insn` must be astral. This is not necessarily the
case since unpaired low/high surrogates may also be encoded in
`U16MatchCharICase32Insn`instructions. We also don't have such an
assert for the case sensitive variant `U16MatchChar32Insn`, which is
generated in almost the same way.

Reviewed By: avp

Differential Revision: D23927543

fbshipit-source-id: 01593e2c434676be22ee4333740b8c448b7276cf

lib/Regex/Executor.cpp

@@ -1048,7 +1048,6 @@ auto Context<Traits>::match(State<Traits> *s, bool onlyAtStart)
 
         case Opcode::U16MatchCharICase32: {
           const auto *insn = llvh::cast<U16MatchCharICase32Insn>(base);
-          assert(insn->c >= 0x010000 && "Character should be astral");
           bool matched = false;
           if (!c.atEnd()) {
             CodePoint cp = c.consumeUTF16();

test/hermes/regexp_unicode.js

@@ -89,6 +89,13 @@ print(/.*/u.exec("\u0101bc\ndef")[0].length);
 // We should not match a low surrogate in a Unicode regexp.
 print(!! /\uDE42/u.exec("\uD83D\uDE42ZZZ"));
 // CHECK-NEXT: false
+// We should match an unpaired surrogate.
+print(!! /\uDC00/u.exec("\uDC00"));
+// CHECK-NEXT: true
+// Test the case insensitive variant.
+print(!! /\uDC00/iu.exec("\uDC00"));
+// CHECK-NEXT: true
+// We should match the low surrogate when Unicode is off.
 print(!! /\uDE42/.exec("\uD83D\uDE42ZZZ"));
 // CHECK-NEXT: true