Tracking issue for constant folding

[lachlansneff]

This is the tracking issue for minimal constant-folding support. The aim is to collapse code like

v0 = iconst.i32 5
v1 = iconst.i32 37
v2 = iadd v1, v0

into

v2 = iconst.i32 42

and

ebb0:
  v0 = bconst true
  brnz v0, ebb1
  ...
ebb1
  ...

into

ebb0:
  jump ebb1
  ...
ebb1
  ...

Progress:

• Add constant folding pass

• Collapse simple operations on constants.

• Collapse branches when branch can be determined at compile time.