[ELF][MIPS] Remove applying the redundant bit-mask
[ELF][MIPS] Remove applying the redundant bit-mask

The `mipsHigh` return type is `uint16_t` so we do not need to extract
low 16-bits from return value explicitly.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@255622 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld