[Mips] Use endian::read/write function to load/save MIPS32 instructions

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222467 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Remove unused function argument

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222468 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Remove duplicated relocation calculation routines

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222469 91177308-0d34-0410-b5e6-96231b3b80d8
[Mips] Simplify the code calculates HI16/LO16 relocations

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222470 91177308-0d34-0410-b5e6-96231b3b80d8