[AArch64][NEON] NEON intrinsic compilation error with -fno-lax-vector-conversion flag fix (#149329)

Issue originally raised in
#71362 (comment).
Certain NEON intrinsics that operate on poly types (e.g. poly8x8_t)
failed to compile with the -fno-lax-vector-conversions flag. This patch
updates NeonEmitter.cpp to insert an explicit __builtin_bit_cast from
poly types to the required signed integer vector types when generating
lane-related intrinsics. A test 'neon-bitcast-poly.ll' is included.

Author: Amichaxx

clang/test/CodeGen/AArch64/neon-scalar-copy.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple arm64-none-linux-gnu -target-feature +neon \
+// RUN: %clang_cc1 -triple arm64-none-linux-gnu -target-feature +neon -flax-vector-conversions=none\
 // RUN: -disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
 
 // REQUIRES: aarch64-registered-target || arm-registered-target

clang/test/CodeGen/AArch64/neon-vget.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple arm64-apple-darwin -target-feature +neon \
+// RUN: %clang_cc1 -triple arm64-apple-darwin -target-feature +neon -flax-vector-conversions=none \
 // RUN:   -disable-O0-optnone -emit-llvm -o - %s \
 // RUN: | opt -S -passes=mem2reg | FileCheck %s
 

clang/test/CodeGen/AArch64/poly64.c

@@ -1,5 +1,5 @@
 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 5
-// RUN: %clang_cc1 -triple arm64-none-linux-gnu -target-feature +neon \
+// RUN: %clang_cc1 -triple arm64-none-linux-gnu -target-feature +neon -flax-vector-conversions=none\
 // RUN:  -ffp-contract=fast -disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg,sroa \
 // RUN:  | FileCheck %s
 

clang/utils/TableGen/NeonEmitter.cpp

@@ -1401,14 +1401,12 @@ void Intrinsic::emitBodyAsBuiltinCall() {
       if (LocalCK == ClassB || (T.isHalf() && !T.isScalarForMangling())) {
         CastToType.makeInteger(8, true);
         Arg = "__builtin_bit_cast(" + CastToType.str() + ", " + Arg + ")";
-      } else if (LocalCK == ClassI) {
-        if (CastToType.isInteger()) {
-          CastToType.makeSigned();
-          Arg = "__builtin_bit_cast(" + CastToType.str() + ", " + Arg + ")";
-        }
+      } else if (LocalCK == ClassI &&
+                 (CastToType.isInteger() || CastToType.isPoly())) {
+        CastToType.makeSigned();
+        Arg = "__builtin_bit_cast(" + CastToType.str() + ", " + Arg + ")";
       }
     }
-
     S += Arg + ", ";
   }