Revert "GlobalISel: Make LLT constructors constexpr"

This reverts commit 5a95be22.

It causes GCC 5.3 to segfault:

In file included from /work/llvm.monorepo/llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp:357:0:                                               lib/Target/AArch64/AArch64GenGlobalISel.inc:189:17:   in constexpr expansion of ‘llvm::LLT::scalar(16u)’
lib/Target/AArch64/AArch64GenGlobalISel.inc:205:1: internal compiler error: Segmentation fault