[SYCL] Make invoke_simd convert its arguments to appropriate type. (#6544)

* [SYCL] Make invoke_simd convert its arguments to appropriate type.

- Introduce an intermediate lambda in invoke_simd and call SIMD target
from the lambda with given (SIMD) arguments coming from the lambda's
formal paremeters. This way compiler automatically performs necessary
argument type conversion.

- Introduce a new implicit constructor for simd to allow conversion
of simd objects with _VecExt storage kind (used in invoke_simd
extension).

- Protect linkonce_odr invoke_simd targets from DCE after splitting out ESIMD
part where there are no references to them by changing linkage to WeakODR.

This new scheme complicates the SIMD target function pointer flow, so
LowerInvokeSimd.cpp update is needed to accomodate the new flow.

Signed-off-by: Konstantin S Bobrovsky <[email protected]>

Author: kbobrovs

llvm/include/llvm/SYCLLowerIR/LowerInvokeSimd.h

@@ -24,4 +24,8 @@ class SYCLLowerInvokeSimdPass : public PassInfoMixin<SYCLLowerInvokeSimdPass> {
 
 ModulePass *createSYCLLowerInvokeSimdPass();
 void initializeSYCLLowerInvokeSimdLegacyPassPass(PassRegistry &);
+
+// Attribute added to functions which are known to be invoke_simd targets.
+constexpr char INVOKE_SIMD_DIRECT_TARGET_ATTR[] = "__invoke_simd_target";
+
 } // namespace llvm