[ICP] Handling must tail calls in indirect call promotion

Per the IR convention, a musttail call must precede a ret with an optional bitcast. This was violated by the indirect call promotion optimization which could result an IR like:

    ; <label>:2192:
      br i1 %2198, label %2199, label %2201, !dbg !226012, !prof !229483

    ; <label>:2199:                                   ; preds = %2192
      musttail call fastcc void @foo(i8* %2195), !dbg !226012
      br label %2202, !dbg !226012

    ; <label>:2201:                                   ; preds = %2192
      musttail call fastcc void %2197(i8* %2195), !dbg !226012
      br label %2202, !dbg !226012

    ; <label>:2202:                                   ; preds = %605, %2201, %2199
      ret void, !dbg !229485

This is being fixed in this change where the return statement goes together with the promoted indirect call. The code generated is like:

    ; <label>:2192:
      br i1 %2198, label %2199, label %2201, !dbg !226012, !prof !229483

    ; <label>:2199:                                   ; preds = %2192
      musttail call fastcc void @foo(i8* %2195), !dbg !226012
      ret void, !dbg !229485

    ; <label>:2201:                                   ; preds = %2192
      musttail call fastcc void %2197(i8* %2195), !dbg !226012
      ret void, !dbg !229485

Differential Revision: https://reviews.llvm.org/D79258