Revert "Merge memtag instructions with adjacent stack slots."

*** Bad machine code: Tied use must be a register ***
- function:    stg_alloca17
- basic block: %bb.0 entry (0x20076710580)
- instruction: early-clobber %0:gpr64common, early-clobber %1:gpr64sp = STGloop 272, %stack.0.a :: (store 272 into %ir.a, align 16)
- operand 3:   %stack.0.a

http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/21481/steps/test-check-all/logs/stdio

This reverts commit b675a762.