Simplify argument capture loop for bitcode generation.

[kquick]

[NOTE: this PR is actually based on PR #48 and if that PR is updated, I will rebase this branch and update this PR]

Fewer conditional tests, avoids re-computation of values, uses more direct conversion of values between types, and merges adjacent blocks for more direct handling.

Adds a BitcodeError::MissingArgValue error for handling arguments where build-bom expected a following value but none was present.