[mips] Implement the following optimizations using dominance information to

[mips] Implement the following optimizations using dominance information to
make PIC calls a little more efficient:

1. Remove instructions setting up $gp if it is known that a function has been
   called at least once.
2. Save the address of a called function in a register instead of loading
   it from the GOT at every call site.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195892 91177308-0d34-0410-b5e6-96231b3b80d8