[Hexagon] Silence warnings with MSVC
Fixes:
```
[3400/4469] Building CXX object lib\Target\Hexagon\CMakeFiles\LLVMHexagonCodeGen.dir\HexagonVectorCombine.cpp.obj
C:\git\llvm-project\llvm\lib\Target\Hexagon\HexagonVectorCombine.cpp(2698): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
C:\git\llvm-project\llvm\lib\Target\Hexagon\HexagonVectorCombine.cpp(2724): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
```
