llvmpipe: assert triggers in LLVM

Since !4122 (merged), I'm seeing the following assert triggered when running wglgears with llvmpipe on Windows with LLVM 10:

Assertion failed!

Program: ...ma\build\mesa-demos-x64\src\wgl\Debug\OPENGL32.dll
File: C:\Users\kusma\source\repos\llvm-project\...\Globals.cpp
Line: 399

Expression: InitVal->getType() == getValueType() && "Initializer type must match GlobalVariable type"

The first bad commit is 38e81938.

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