[mips] Store 64-bit `li.d' operand as a single 8-byte value

Now assembler generates two consecutive `.4byte` directives to store
64-bit `li.d' operand. The first directive stores high 4-byte of the
value. The second directive stores low 4-byte of the value. But on
64-bit system we load this value at once and get wrong result if the
system is little-endian.

This patch fixes the bug. It stores the `li.d' operand as a single
8-byte value.

Differential Revision: https://reviews.llvm.org/D68778

llvm-svn: 374598