[IR] Convert some operands to instructions

* Strings that were built from pieces were being represented as
  a compound string and used in building non-static regexps.

* Converted them to individual instructions and left Regexp operand
  representing a static regexp. Left a FIXME in IRBuilder for detecting
  syntax errors in regexp patterns and adding a syntax error during
  IR building rather than at runtime. This lets us retain these regexps
  as operands that won't throw exceptions when constructed.

* Eliminates debugging errors that were being thrown from the
  interpreter loop.
Delete a bunch of dead code in ThreadContext.