[SVE] Fix inline assembly parsing crash

This patch fixes a crash encountered when compiling this code:

  ...
  float16_t a;
  __asm__("fminv %h[a], %[b], %[c].h"
          : [a] "=r" (a)
          : [b] "Upl" (b), [c] "w" (c))

The issue here is when using the 'h' modifier for a register
constraint 'r'.

Differential Revision: https://reviews.llvm.org/D93537