Temporarily disallow asm blocks from writing an initialized registers

[mohammadfawaz]

This is due to the addition of mem2reg in #3128. Some odd patterns arise when you allow asm blocks to read and write the same value in IR, which causes mem2reg not to behave well. In any case, we should disallow these patterns temporarily until #2819 is resolved and we have a better mental model regarding asm blocks in IR. write an initialized register.

I think this change can be done in the parser or type checking. Example use cases:

Reading and writing the same register in the same instruction
Writing register r1 even though r1 is initialized to ptr.

    asm(r1: ptr) {
        lw r1 r1 i0;
        r1: u64
    }

Reading and writing the same register in different instructions
Writing registers r1 and r2 even though both are initialized.

    asm(r1: ptr1, r2: reg2) {
        lw r2 r1 i0;
        lw r1 r2 i0;
        r1: u64
    }

[otrho]

Checks like these could be permanent IMO, at least in the form of warnings.

[mohammadfawaz]

I spoke the @vaivaswatha, and the real requirement here is that a register cannot be both initialized in the args list of the asm block AND be assigned to in the body of the block. So my examples above are correct, but my description was not accurate and so I just updated it.