[NVPTX] Use MBB.begin() instead MBB.front() in NVPTXFrameLowering::emitPrologue

The second argument of `NVPTXFrameLowering::emitPrologue(MachineFunction &MF, MachineBasicBlock &MBB)` is the first MBB of the MF. In that function, it assumes the first MBB always contains instructions, so it gets the first instruction by MachineInstr *MI = &MBB.front();. However, with the reproducer/test case attached, all instructions in the first MBB is cleared in a previous pass for stack coloring. As a consequence, MBB.front() triggers the assertion that the first node is actually a sentinel node. Hence we are using MachineBasicBlock::iterator to iterate over MBB.

Fix #52623.

Differential Revision: https://reviews.llvm.org/D132663