[llvm] [llvm][WebAssembly] mark BR_TABLE as isIndirectBranch (PR #73391)

[Xu Jun via llvm-commits]

xujuntwt95329 wrote:

> Why do you think `BR_TABLE` is an indirect branch? It takes destination BBs. The only difference from a simple `BR` or `BR_IF` is it takes variable number of them.

I got this description from the WebAssembly spec:
> Branch instructions come in several flavors: 
 `br` performs an `unconditional branch`, 
 `br_if` performs a `conditional branch`, and 
 `br_table` performs an `indirect branch` through an operand indexing into the label vector that is an immediate to the instruction, or to a default target if the operand is out of bounds.

https://webassembly.github.io/spec/core/syntax/instructions.html#control-instructions

https://github.com/llvm/llvm-project/pull/73391