LLVM: elide some loads when lowering

[andrewrk]

Generally, the load instruction may need to make a copy of an
isByRef=true value, such as in the case of the following code:

pub fn swap(comptime T: type, a: *T, b: *T) void {
    const tmp = a.*;
    a.* = b.*;
    b.* = tmp;
}

However, it only needs to do so if there are any instructions which can
possibly write to memory. When calling functions with isByRef=true
parameters, the AIR code that is generated looks like loads followed
directly by call.

This allows for a peephole optimization when lowering loads: if the load
instruction operates on an isByRef=true type and dies before any side effects
occur, then we can safely lower the load as a no-op that returns its
operand.

This is one out of three changes I intend to make to address #11498.
However I will put these changes in separate branches and merge them
separately so that we can have three independent points on the perf
charts.