[ELF][MIPS] Use MIPS R6 `sigrie` as a trap instruction

[ELF][MIPS] Use MIPS R6 `sigrie` as a trap instruction

Current value using as a trap instruction (0xefefefef) is not a good choice
for MIPS because it's a valid MIPS instruction `swc3 $15,-4113(ra)`. This
patch replaces 0xefefefef by 0x04170001. For all MIPS ISA revisions before
R6, this value is just invalid instruction. Starting from MIPS R6 it's
a valid instruction `sigrie 1` which signals a Reserved Instruction exception.

mips-traps.s test case is added to test trap encoding. Other test cases
are modified to remove redundant checking.

Differential revision: https://reviews.llvm.org/D54154

llvm-svn: 347029