IR: Don't add inbounds to GEPs of extern_weak variables

Global variables that have `extern_weak` linkage may be null, so it's
incorrect to add `inbounds` when constant folding.

This also fixes a bug when parsing global aliases, whose forward
reference placeholders are global variables with `extern_weak` linkage.
If GEPs to these aliases are encountered before the alias itself, the
GEPs would incorrectly gain the `inbounds` keyword as well.
UseListOrder: Correctly count the number of uses

This is an off-by-one bug I found by inspection, which would only
trigger if the bitcode writer sees more uses of a `Value` than the
reader.  Since this is only relevant when an instruction gets upgraded
somehow, there unfortunately isn't a reasonable way to add test
coverage.
BitcodeReader: Only create one basic block for each blockaddress

Block address forward-references are implemented by creating a
`BasicBlock` ahead of time that gets inserted in the `Function` when
it's eventually encountered.

However, if the same blockaddress was used in two separate functions
that were parsed *before* the referenced function (and the blockaddress
was never used at global scope), two separate basic blocks would get
created, one of which would be forgotten creating invalid IR.

This commit changes the forward-reference logic to create only one basic
block (and always return the same blockaddress).