[mips] Move initGlobalBaseReg to MipsFunctionInfo. NFC

Move initGlobalBaseReg from MipsSEDAGToDAGISel to MipsFunctionInfo.
This way functions used for handling position independent code during
instruction selection, getGlobalBaseReg and initGlobalBaseReg,
end up in same class.

Differential Revision: https://reviews.llvm.org/D62586

llvm-svn: 362206