[NVVM] Support - Followup enhancements

[abhilash1910]

Description

Issue Link - #981

Changes to be addressed in this WIP PR:

• LTO IR testing
• Is there a way to add multiple modules?
  {If / when it is possible to add multiple modules, a test with code that uses something from libdevice is probably a good idea.
  It's also useful to be able to lazily add a module}
• apply bitcode pattern input for libnvvm

cc @leofang

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