Revert^2 "ARM64: toHalf() intrinsic for ARMv8"

This reverts commit 67bf99b8a575b15c117a3fbf7aae421330e4795a.

Reason for revert: The original CL was reverted because
of build breakages not directly related to the CL itself.

Change-Id: Ic98e9912701d81d73bc3af719a7e3a8e44e8c058
diff --git a/compiler/optimizing/intrinsics_x86.cc b/compiler/optimizing/intrinsics_x86.cc
index 5a622ca..95aa4c0 100644
--- a/compiler/optimizing/intrinsics_x86.cc
+++ b/compiler/optimizing/intrinsics_x86.cc
@@ -3082,6 +3082,7 @@
 UNIMPLEMENTED_INTRINSIC(X86, CRC32UpdateBytes)
 UNIMPLEMENTED_INTRINSIC(X86, CRC32UpdateByteBuffer)
 UNIMPLEMENTED_INTRINSIC(X86, FP16ToFloat)
+UNIMPLEMENTED_INTRINSIC(X86, FP16ToHalf)
 
 UNIMPLEMENTED_INTRINSIC(X86, StringStringIndexOf);
 UNIMPLEMENTED_INTRINSIC(X86, StringStringIndexOfAfter);