[AArch64] Improve orr+movk sequences for MOVi64imm.

The existing code has three different ways to try to lower a 64-bit
immediate to the sequence ORR+MOVK.  The result is messy: it misses
some possible sequences, and the order of the checks means we sometimes
emit two MOVKs when we only need one.

Instead, just use a simple loop to try all possible two-instruction
ORR+MOVK sequences.

Differential Revision: https://reviews.llvm.org/D47176

llvm-svn: 333218