[utils] Reflow asm check generation to tolerate blank lines

This change introduces two fixes. The second fix allows to generate
a test to check the first fix.

- Output `CHECK-EMPTY` prefix for an empty line in ASM output. Before that
  fix `update_llc_test_checks.py` incorrectly emits `CHECK-NEXT: <space>`
  prefix.
- Fix the `ASM_FUNCTION_MIPS_RE` regex to stop on a real function
  epilogue not on an inline assembler prologue and include inline
  assembler code into a test.

Differential Revision: https://reviews.llvm.org/D47192