Add Triple::isMIPS()/isMIPS32()/isMIPS64(). NFC

There are quite a few if statements that enumerate all these cases. It gets
even worse in our fork of LLVM where we also have a Triple::cheri (which
is mips64 + CHERI instructions) and we had to update all if statements that
check for Triple::mips64 to also handle Triple::cheri. This patch helps to
reduce our diff to upstream and should also make some checks more readable.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D48548

llvm-svn: 335493
diff --git a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
index fb22071..3150057 100644
--- a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -513,11 +513,7 @@
     CpRestoreOffset = -1;
 
     const Triple &TheTriple = sti.getTargetTriple();
-    if ((TheTriple.getArch() == Triple::mips) ||
-        (TheTriple.getArch() == Triple::mips64))
-      IsLittleEndian = false;
-    else
-      IsLittleEndian = true;
+    IsLittleEndian = TheTriple.isLittleEndian();
 
     if (getSTI().getCPU() == "mips64r6" && inMicroMipsMode())
       report_fatal_error("microMIPS64R6 is not supported", false);