LLVM intermediate code instrumented by MemorySanitizer can take an
inordinate amount of time to compile into machine code when using the
llvm::CodeGenOpt::Default optimization level.

While lower optimization levels can make the generated code slower to
execute, the 'None' level is just 5-20% slower, which is negligible
compared to the slowdown from the instructions added by the MSan
instrumentation itself.

llvm::CodeGenOpt::Default:

-------------------------------------------------
Benchmark                                    Time
-------------------------------------------------
ClearImage/VK_FORMAT_R8G8B8A8_UNORM      0.912 ms
ClearImage/VK_FORMAT_R32_SFLOAT           3.83 ms
ClearImage/VK_FORMAT_D32_SFLOAT          0.866 ms
Triangle/Hello                           0.996 ms
Triangle/Multisample                      4.63 ms

llvm::CodeGenOpt::None:

-------------------------------------------------
Benchmark                                    Time
-------------------------------------------------
ClearImage/VK_FORMAT_R8G8B8A8_UNORM      0.974 ms
ClearImage/VK_FORMAT_R32_SFLOAT           5.42 ms
ClearImage/VK_FORMAT_D32_SFLOAT          0.937 ms
Triangle/Hello                            1.16 ms
Triangle/Multisample                      4.70 ms

Bug: b/173257647
Change-Id: I0d14bf1834ce23045409fa162153991cabbeac3c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50448
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>