swr: Fix build with llvm-10.0.

swr: Fix build with llvm-10.0.

Fix build error after llvm-10.0 commit ("1b2842bf902a [Alignment][NFC]
CreateMemSet use MaybeAlign").

../src/gallium/drivers/swr/swr_shader.cpp: In member function ‘void (* BuilderSWR::CompileGS(swr_context*, swr_jit_gs_key&))(HANDLE, HANDLE, SWR_GS_CONTEXT*)’:
../src/gallium/drivers/swr/swr_shader.cpp:738:65: error: no matching function for call to ‘BuilderSWR::MEMSET(llvm::Value*&, llvm::Constant*, int, long unsigned int)’
       MEMSET(pStream, C((char)0), VERTEX_COUNT_SIZE + CONTROL_HEADER_SIZE, sizeof(float) * KNOB_SIMD_WIDTH);
                                                                 ^
In file included from ../src/gallium/drivers/swr/rasterizer/jitter/builder.h:163:0,
                 from ../src/gallium/drivers/swr/swr_shader.cpp:43:
src/gallium/drivers/swr/rasterizer/jitter/gen_builder.hpp:51:11: note: candidate: llvm::CallInst* SwrJit::Builder::MEMSET(llvm::Value*, llvm::Value*, uint64_t, llvm::MaybeAlign, bool, llvm::MDNode*, llvm::MDNode*, llvm::MDNode*)
 CallInst* MEMSET(Value *Ptr, Value *Val, uint64_t Size, MaybeAlign Align, bool isVolatile = false, MDNode *TBAATag = nullptr, MDNode *ScopeTag = nullptr, MDNode *NoAliasTag = nullptr)
           ^
src/gallium/drivers/swr/rasterizer/jitter/gen_builder.hpp:51:11: note:   no known conversion for argument 4 from ‘long unsigned int’ to ‘llvm::MaybeAlign’
In file included from ../src/gallium/drivers/swr/rasterizer/jitter/builder.h:163:0,
                 from ../src/gallium/drivers/swr/swr_shader.cpp:43:
src/gallium/drivers/swr/rasterizer/jitter/gen_builder.hpp:56:11: note: candidate: llvm::CallInst* SwrJit::Builder::MEMSET(llvm::Value*, llvm::Value*, llvm::Value*, llvm::MaybeAlign, bool, llvm::MDNode*, llvm::MDNode*, llvm::MDNode*)
 CallInst* MEMSET(Value *Ptr, Value *Val, Value *Size, MaybeAlign Align, bool isVolatile = false, MDNode *TBAATag = nullptr, MDNode *ScopeTag = nullptr, MDNode *NoAliasTag = nullptr)
           ^
src/gallium/drivers/swr/rasterizer/jitter/gen_builder.hpp:56:11: note:   no known conversion for argument 4 from ‘long unsigned int’ to ‘llvm::MaybeAlign’

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>