We'd introduced separate versions of `llvm.dx.resource.load` with a
struct return to handle the CheckAccessFullyMapped case without making
the IR for the common case unnecessarily complicated. However, at this
point the common case is really `resource.getpointer`, so the ergonomics
of a simplified version of `load` don't actually gain us as much as the
cost of having multiple opcodes.
Drop the `dx.resource.loadchecked` functions and have `dx.resource.load`
consistently return `{element_type, i1}`.
2c7c07df82