gallivm, draw: fix some LLVM non-opaque pointers
As LLVM 15 transitions to opaque pointers, we need to update the use of LLVM deprecated methods (e.g. LLVMBuildGEP) to the methods taking an explicit type (e.g. LLVMBuildGEP2).
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As LLVM 15 transitions to opaque pointers, we need to update the use of LLVM deprecated methods (e.g. LLVMBuildGEP) to the methods taking an explicit type (e.g. LLVMBuildGEP2).